I pulled a grill over the weekend and ours was really nothing more than the same open cell foam you'd find if you're used to the larger AC grill mounted on the ceiling. It did have a cardboard border ring but it wasn't a part of a filter assembly, just looked to be a spacer.
I'm sure theres a cold air return filter out there just waiting to be used in an RV. Might even look for small thread inserts to use in place of the wood screws holding the grate to the ceiling.
2014 Eagle Premier 331RETS
2016 Silverado 3500 DuraMax/Allison
1975 Corvette & a couple ATV's