I had a 1995 Peterbilt 377 tractor with a 425 hp Detroit engine. It weighed 18,500 lbs bobtail (no trailer) It had amazing power and would get 14 mpg without a trailer. (6 2/3 mpg with a GCVW of 80,000 lbs). And it had two 125 gallon fuel tanks.
The reason I say this is if you could find a used highway tractor with a long enough wheel base to mount a 21' trailer body, it would be pretty economical. Better yet, find a 53' drop deck flatbed trailer and you could build a log cabin on it. Then license
it as an RV and not be subject to any commercial vehicle laws (abuse). That would really P them off at the weight