There are a lot of options out there for better speakers, but it seems car audio sizes and what the RV builders use are two different things.
I upgraded my trailer's speakers to some Pioneer 6" two-way. Despite the stock speakers were also six in, the Pioneer do not fit perfect in the hole in the ceiling.
First off, the Pioneer have real magnets on them, not a fridge magnet like the factory, so they're several inches deeper. Secondly, the Pioneer use a heavy steel frame to support the cone and magnet, which prevents the speaker fitting flush into the hole.
One of these days I need to cut spacers from some balsa wood or something, to fill the 1/4" gap between the speaker's bezel and the ceiling in the RV.
Oh heck, I'll throw in some pictures too:


I added some thump to my trailer (reliving the missed opportunity of youth to have a big stereo in my high school ride.)
