I used to have a problem with that wih the side-saddle tanks on my camper. A fill cap on both sides. I'd have to warn the attendant in the booth that I'd have to pull out and turn around so hesheit wouldn't think I was cutting out on paying. I think it was 40-45 gallons for a total fill. (And don't forget to switch the valve back to the main tank.) Those side saddle tanks were like the Pinto gas tank, vulnerable to collisions but on the sides instead.
Terry
Itty bitty camper on great big Pawnee Grasslands