Here's a VERY CHEAP alternative for the air-conditioner filter.
The Coleman-Mach air handler in my RV uses Coleman-Mach Foam Air Conditioner Filters part #8330A3961
The dimensions of the foam filter are: 13.5" x 6" x 0.25" thick
The price is $9.99 plus tax plus shipping.
Coleman-Mach recommends replacing the air filter annually or every 90 days of continuous use. I was washing mine in the sink until it started crumbling and falling apart into dust.
Someone on another RV forum recommended getting the $1 window air-conditioner foam air filters at Walmart and cut them to size.
Here they are:
Duck Foam Air Conditioner Filter (Walmart in-store only)
24" x 15" x 0.25" thick
Cost: $0.81 cents plus tax!
I cut (4) filters from one package. The foam is exactly the same thickness, and maybe a little more dense but I don't suspect return air flow will be that much impeded, it's made to work for a window a/c and it should work fine.
I bought (5) packs @ $0.81 cents each for $4.29 with tax total, which will make (20) filters at a
cost of $0.22 cents each!