For years we had auxillary gas tanks. In fact they were so popular Ford & GM offered them in their PUs. I think the aux tanks were better than transfer tanks which are only legal for diesel only. If I went back to towing 5er or TT I think I would have a tank built for my PU bed and have a valve that tapped into my filler neck. Illegal I suppose but it is a PITA stopping every few miles for gas. MHs carry about 80 gallons of fuel so it isn't the amount of fuel carried.Mostly everyone today is afraid of being sued and smog testing.