I'm confused... is the OP wanting a portable generator that is dual fuel or triple fuel, or is a built in genset something he wants?
A portable generator is one thing. However, if he wants to go the Onan route with a fuel tank under the rig, I'd go with the same fuel that the tow vehicle uses. There are some places which can install a gasoline tank underneath a trailer, weld on a place an Onan generator could go, then wire it in with an ATS near the RV converter... doing the job up to building fire/electrical codes and state/federal DOT codes. An advantage of this setup is that the gasoline tank installed under the trailer can be used with a fuel pump to give an auxiliary tank for the tow vehicle, similar to how some toy haulers are set up.
I thought about having this done, but the upfit cost was fairly steep (north of $4500) because of all the custom fab work involved (having it installed is one thing... having it installed with shock mounts, acoustic deadening, exhaust resonators and baffles to reduce vibration is another.) However, having 25-100 gallons of fuel for a generator ready to go at a push of a button on a boondocking trip is a pretty nice thing.
