I suspect something is wrong with your math.
Assuming a 60mph cruise speed:
- at 6MPG, you are burning 10GPH
- If you lose 1 MPG, that means you are burning 12GPH
- So the generator is using 2GPH.
2GPH is a lot for a generator to burn. I'm not sure about your Onan but my Yahaha 2400 running the air/con continously gets about 5hrs/gal or about 0.2GPH, which wouldn't be enough to measure in terms of the MH MPG.
If true, your 24gal propane tank is only going to be good for a single night running the generator.
Assuming more realistic fuel consumption, I would go with the gas option. With the proliferation of pre-filled propane tanks, fewer and fewer places actually fill propane tanks, so it's a pain to deal with.
