Unless the genny has an electric starter you'll need to mount it somewhere accessible so you can get at the pull starter. A rear mounted "back porch" would work but leaves the thing out there to be stolen. An undercarriage box with a door in the bus skirt would be great but requires a bit of cutting and welding.
Trojan is proven name for deep cycle golf cart batteries but others are also good (interstate, etc).
The back porch thing could be used for the dirt bike also.