T18skyguy, Don't know about fuel tanks designed to be 80% filled, except for the LP tank. I know on my gasser with a 75 gallon tank, I had to put in just over 70 gallons one time. It wasn't empty, but a lot closer than I ever want to be again.
This may not work for every case but here is what I did. I had a 30 amp coach with a 5.5KW generator. When on shore power I could only run one AC unit (front or back). With the genny I could run both AC's. The Genny had two breakers, a 30A and a 20A. The 30A served the coach and the 20A served only the back AC unit. I opened the junction box from the genny, traced the 20A circiut, and cut the line to the AC, installed a male plug on the line from the 20A breaker and a female soccet on the line to the AC. Now I can separate the plug, and connect an extension cord to the line to the back AC and plug into a 20A circuit at the power box. That way I can run both AC's from shore power or by re-connecting the plug to the Genny, run both on the Genny. Hope all this makes some sense. If you don't understand AC power, get help. Disclaimer: I didn't think this up on my own. I read about it some where on this forum.