Other than not being able to use the bed, this might almost be Uwe-approved.
LOL -- I could put enough auxiliary tankage in the bed of my diesel truck (while still able to close the toneau cover) that it could do the whole Cannonball without ever stopping for fuel. Stock tank: 33 gallons. An extra 100 would do the trick at 20 mpg, and I normally get 27.
However, to set a (diesel) record, I'd have to average a bit over 104 mph, and I'm not sure what the consumption would be at that speed. Better make it 150 gallons. Three 55 gallon drums, on their sides, in a wood frame -- dirty deeds, done dirt cheap.