I have 3 roof top ac units. I checked the inside coils and they are very nasty. It appears that the cheap sponge like factory filters don't work well. I do keep them clean but too much junk gets by th...
I just bought filters at an RV dealer in Las Vegas (Johnnie Walker)and they are only like three dollars. Out of one you cut it in two and you have filters for both air conditioners. They work pretty good on my air conditioner/heat pump units.
But then again my coach is pretty well sealed and not much dust gets inside the coach. Some coaches have very poor seals around the slides etc.