HEPA filters would be the worst possible thing to add. First of all they might be labeled "HEPA", but a true HEPA is going to be 5+ inches thick and the MERV rating should be 17. Secondly, don't do anything to further restrict the airflow on an RV that are usually very deficient to start with. The blower can only output the amount of air it can take in. I personally use these:
https://www.amazon.com/FilterTime-Al...%2C225&sr=8-14
You'll have to cut to size using the original for a template. they're made of polyester and I put mine in the washer on gentle cycle. If you want a bit more filtration, spray them with Pledge when installing them. These filters job is to keep large particles out of the evaporator coil, cleaning the interior air is just a side effect.
If you feel you need additional interior air filtration, get a floor standing model, they make them rated for HEPA.