I have procrastanated on checking the filters on my two evaporators for my OTR air because I didn't know exactly what was involved in getting to them. Today, I looked at both units and realized it wasn't nearly as complicated as I thought it would be. There are no filters on the evaporators, but the fins were clean. Don't know how or why, and I could easily add a filter but don't think I will.
For those with a CC of my vintage, I have attached a couple pictures to show what is under the head of the bed (OTR Air and Webasto), and the second OTR unit that is under the kitchen cabinet. The bedroom required removing the headboard, mattress and plywood cover over the units. The kitchen required removing two doors and two screws in the bottom shelf.