I have used old work flanged shallow boxes that I got at Home Depot
in my Excella. They will fit even in the exterior walls where space is at a premium and allow use of standard home outlets instead of those strange RV outlet/cover combos.
You will have to make the hole slightly bigger. That is, you will have to make it a neat rectangular opening instead of the jagged, oblong hole that they made at the factory since the cover would hide their work.
It's not a big deal to do, and you are done with looking for those weird outlets, at least there.