nopthing wrong with a 318 at all - Ive had a few from different years in different applications and theyve always worked well for me - Im obviously a mopar advocate - I hate when I see an ol dodge/plymouth rod where someone has gone and stuffed an SBC between the rails - just a pet peeve of mine

youll want to scrap the water pump and intake manifold for something more suited to your application (unless I misunderstood and your building a winnie hot rod :")

Youll have a good set of heads (should be the larger port versions) and the camshaft should be decent for a street machine as well (this all is assuming this was the factory built motor for the motorhome)