Look, I get it. You’re staring at that filthy foam slab that passes for an air filter in your camper, wondering if it’s even doing anything. The original is probably gray with dust, maybe growing something you don’t want to think about. The dealer wants $25 plus shipping for a piece of foam. That’s the racket.
So you search Amazon, find a six-pack for ten bucks, and think, “This has to be garbage.” I had the same thought. Then I bought them anyway because I’m cheap and desperate. Turns out, sometimes the cheap gamble pays off.
Here’s the deal with the Acostop 6-pack of 0.5” x 7.5” x 14” 40PPI filters. They’re cut for Dometic units, specifically the ones that take part numbers 3313107.103 and 3105012.003. If you’ve got a Duo-Therm or a Penguin, this is your size. But even if you don’t, these things are easy to trim, which I’ll get into in a minute.
Why You Should Care About a Dumb Foam Filter
Before we talk about the filters themselves, let’s address why you’re even here. Your RV AC pulls in outside air, runs it over the coils, and blows it back inside. That foam filter is the only thing standing between the great outdoors and your lungs. Dust, pollen, road grime, that weird cottonwood fluff that gets everywhere — all of it ends up on that filter.
When that filter clogs, your AC works harder. It moves less air. It freezes up on hot days because the coils get too cold with restricted airflow. I’ve seen it happen. A buddy of mine fried a compressor because he ignored the filter for two seasons. The filter was so packed with dust it felt like felt. Don’t be that guy.
The other thing nobody talks about: that filter gets gross in ways you don’t want to breathe. Mold spores, mildew, bacteria from all that moisture. If you’re sensitive to that stuff, you’ll feel it the moment you turn the AC on after storage. A fresh filter fixes that instantly.
First Impressions and What You’re Actually Getting
The package shows up and it’s exactly what you’d expect — six pieces of open-cell polyurethane foam wrapped in plastic. Nothing fancy. No branded box, no QR codes to scan, no “register your product” nonsense. Just foam.
Each piece measures 0.5 inches thick, 7.5 inches wide, and 14 inches long. That half-inch thickness matters. Some cheap replacements come in at 0.25 inches and they don’t seal worth a damn. Air just flows around them and you’re back to square one. These actually seat properly in the filter slot.
The 40 PPI rating means 40 pores per inch. That’s the sweet spot for RV AC filters. It catches the stuff you don’t want while still letting enough air through to keep the unit efficient. I’ve seen 20 PPI filters that might as well be screen mesh, and 80 PPI ones that choke airflow. These sit right in the middle where they should be.
Pull one out of the package and it springs back to shape immediately. That’s a good sign. Cheap foam that stays compressed when you squeeze it is dead foam. This stuff has some life to it.
The Installation Reality
If you’ve got a Dometic unit, installation takes about thirty seconds. Pop the ceiling grille off (usually four screws or a quarter-turn latch), pull the old filter out of the slot, slide the new one in, snap the grille back. Done.
The foam is slightly oversized compared to the filter slot in some units, which is actually a good thing. You want a tight fit. A filter that’s loose in the frame is a filter that’s letting unfiltered air bypass it. When I slid mine in, it compressed just enough to seat firmly against the edges of the frame. No gaps, no rattling, no air sneaking around the sides.
If you don’t have a Dometic and you’re wondering whether these will work for your rig — maybe you’ve got a Coleman or one of those oddball units — grab a utility knife and a straight edge. The foam cuts clean. Mark your dimensions, press down with the blade, and you get a perfect edge. I trimmed one for a friend’s older Coleman unit and it took maybe two minutes. The foam doesn’t crumble or tear like some of the cheaper stuff.
While you’re in there, take a look at the evaporator coils. If your filter was overdue, the coils underneath are probably dirty too. A soft brush and some compressed air will do wonders. I wrote about the full maintenance routine in my piece on RV A/C Filters Replacement, 16 x 40 x 1/4 Inches RV Air Conditioner Replacement Filters — different product, same principle. Worth a read if you want to maximize the lifespan of your unit.
The Real World Test
Numbers on paper are one thing. Let’s talk about what happens when you actually run this thing.
I put a set in my own rig — a 2019 Keystone Passport with a Dometic Duo-Therm on the roof. The original filter was shot. It had that gray-brown color of accumulated dust, and when I held it up to the light, nothing came through. The AC still blew cold, but it was running longer cycles and the airflow felt weak at the vents.
After swapping in the Acostop filter, the difference was immediate. Airflow at the ceiling vents picked up noticeably. The unit cycled off sooner because it was actually moving air across the coils instead of struggling against a clogged filter. Over the next few weeks, I kept an eye on the new filter. After a month of regular use, including a dusty stretch through New Mexico, the filter showed a visible layer of dust on the intake side. Which is gross, sure, but it means the foam is doing its job. That dust would have been on my coils otherwise.
I also tested these in a friend’s rig that has a smaller AC unit. Same story. The foam compressed to fit the tighter slot without any cutting, and the unit’s been running cleaner since.
The Value Math Nobody Does
Let’s talk money, because that’s why you’re really here.
Dometic genuine filters run anywhere from $15 to $25 each at RV dealerships. Online, they’re cheaper, but you’re still looking at $10 to $15 per filter before shipping. This six-pack costs $9.99. That’s less than two bucks per filter.
Even if you replace your filter every season, this pack lasts you six years. Or, if you’re smart about it, you rotate them. Wash one in mild soap and water, let it dry, swap it back in. With six filters in rotation, you’re set for a decade or more. At that point, the pack has essentially paid for itself dozens of times over.
Some people will tell you to just wash the original filter. And yeah, you can do that once or twice before the foam starts breaking down. But at this price point, why bother? The foam in a washed filter never quite returns to its original shape, and the pores get stretched out. Replacement is cheaper than the water and soap you’d use to clean the old one.
What Could Be Better
Look, I’m not going to pretend these are perfect. They’re not.
The foam is a bit stiffer than the OEM filter from Dometic. Not a problem in practice — it still compresses and seals fine — but if you’re used to the soft, almost spongy feel of the genuine part, you’ll notice the difference. It’s a minor thing, and it doesn’t affect performance, but it’s there.
The six-pack is also more than most people need, which means you’ll have extras sitting around. That’s fine if you’ve got a second AC unit or a friend with a camper, but if you just need one filter, you’re buying five spares. At $9.99 for the whole pack, though, that’s a pretty good problem to have.
One thing I’ll note for the obsessive types: these are white foam, not the gray or black that some manufacturers use. They’ll show dirt faster. But honestly, that’s a feature. You can see when they need replacing, instead of guessing based on airflow alone.
The Fit and Compatibility Question
I mentioned Dometic part numbers 3313107.103 and 3105012.003 earlier. Those cover most of the popular Duo-Therm and Penguin models from the last couple of decades. But Dometic has made a bunch of different AC units over the years, and not all of them use the same filter.
If you’ve got a newer Dometic unit — say, one of the Penguin II models with the curved shroud — measure your existing filter before you order. The 7.5 x 14 inch size is the most common, but there are variations. A quick check of your current filter’s dimensions will tell you if these are going to work.
For the Coleman and other brand units, you’re rolling the dice on the trim-to-fit approach. The foam is thick enough and cuts clean enough that it usually works, but you’ll want to be precise with your measurements. And if your unit takes a filter that’s dramatically different in size — like the huge ones on some roof-mount units — you’re better off with a purpose-built replacement.
Also worth noting for the seasonal campers out there: if you’re storing your rig over the winter, pull the filter out and keep it somewhere dry. Foam that sits in a damp, cold RV all winter is a mold magnet. Toss it in a ziplock bag and bring it inside. Takes ten seconds and saves you from a nasty surprise in the spring.
While I’m on the subject of AC maintenance — if you’re dealing with a unit that’s underperforming beyond just a dirty filter, you might want to check out my review of the SoftStartRV Soft Start for RV Air Conditioner. Different problem, but it’s another one of those upgrades that makes a real difference when you’re running on a small generator or shore power with limited amperage.
The Bottom Line on These Filters
Here’s the thing. I’ve bought cheap RV parts before and regretted it. Cheap hinges that rust, cheap seals that leak, cheap LED lights that flicker and die after a month. I went into this expecting the same from a $10 six-pack of foam filters.
That’s not what happened. These filters fit, they filter, and they hold up. The foam density is right, the dimensions are accurate, and the price is almost absurdly low compared to what you’d pay at a dealer.
If you need a filter for a Dometic unit and you’ve been putting off the replacement because the OEM price is ridiculous, just buy these. At $9.99 for six, you’re paying less for the whole pack than you would for a single genuine filter. And honestly, I can’t tell the difference in performance between these and the Dometic branded ones I’ve used in the past.
The only reason to buy the OEM part is if you’re the type of person who has to have the factory sticker on everything. If you just want your AC to blow cold, clean air and you don’t want to overthink it, this is your filter.
Six filters. Ten bucks. Thirty seconds to install. There’s not much more to say about it. If you pick one up through the links here, I might get a small cut — doesn’t cost you extra and it helps keep the lights on.
Go swap your filter. Your AC will thank you. Your allergies will thank you. And when you see what comes off those coils once you’ve got clean airflow, you’ll wonder why you waited this long.