Is this filter for the air return or for the part of the unit that fits into the ceiling. Some manufacturers say do not use a filter in the return air compartment, so maybe check your book, if you have one. The filter in the ceiling part of our ROO was only a sponge-type material, maybe 1/4" thick or so. If you can't find any manuals on your specific model, suggest just contacting Dometic directly for info.
Oops, I didn't thoroughly read the post above....you should definitely check with Dometic on a filter for that area. Some folks have gone ahead regardless and used one, but believe it would void any warranty you might have if adding one caused a problem.