As said, almost always the dash radios are DIN or double-DIN, or if not the space for them can be adapted very easily to these standards. If you posted a picture of the existing radio, we could tell you what size to look for.
Last year I replaced the factory radio in my Coachmen motorhome (which really wasn't a bad unit; I think the build sheet listed it as the upgraded model) with a $20 special from Wal-mart. The new radio is probably not as high quality overall, but is much more modern: it has an aux in jack, can play music from SD cards or USB drives, and supports bluetooth. On the other hand, it does not play cassette tapes like the old one did, and rather curiously is lacking an AM receiver.