I don't travel very far so I can get to my campground, unhook, and then go get gas.
I see some of you recommend getting gas at half a tank. I don't know about that. For me this would involve stopping every 100 miles or so. Not going to get very far that way.
When not towing and just driving around in my truck I know I need gas when the little red light next to the gas gauge pops on. When that happens I have about 20 miles to go.
I would not do that towing !! But, I think looking for gas at 1/4 tank would be what I would personally do. However, again, I don't travel far. There may be places in the country where the distance between gas stations is further apart then around here.