Add spawns to mapgen.cpp, Cleanup vehtypedef.cpp

Okay for now I'm going to just fit this in with the current vehicle
system instead of reworking half the game.  Added the RV to mapgen.cpp,
it rarely shows up with the broken vehicles on roads, and will very
rarely be in a parking lot.  Changed all the floor trunks to aisles,
still has 4 trunks so I think it's good for storage.  This still uses
the underbody wide wheels, which is a new part but it doesn't change any
other parts so I think it's okay.
I'd like this to have a chance to spawn at hotels, but I'm not sure how
to do that so for now it doesn't.  If anyone has any tips on that let me
know!