we have had an in bed. 85 gallon fuel tank with transfer pump for over 30 years. I made the tanks, moved it to each of our trucks. always 8' bed , so 120 gal loaded.
In summer when we owned the camp it carried gasoline for lawnmowers, skid loader, leaf bower and such.
Winters when we snowbirded, diesel. Gives us a 1200 + mi. range without need to refuel.
The issue is not the fuel, its the transfer method. No state i know of prohibits a fuel transfer tank, that would be ridiculous given contractors and farmers that need to refuel machinery.
For far less and the flexibilty to carry different fuels for different seasons, not to mention no need for that spendy 5er slider hitch, 8 ' bed with in bed tank, transfer pump . Legal, range, cost effective, frexibility. no install costs.