I think they hang down below the frame rail. That bothers me although I don't offroad. There are specs on their site for all tanks and they state how much below the tank (I think 3 inches but don't quote me).
2013 Chevrolet Silverado 3500 HD Dually
61 gallon Transfer Flow instead of stock.
37 gallon Transfer Flow in bed
Pace-Edwards Jack Rabbit Full Metal tonneau cover
B&W Turnoverball Gooseneck / companion hitch