As PackRat stated we use a two bag system....sometimes take out the back seat of the F350 and add a base with one rack above it but the best we've come up with was to have a hitch (2017 F350 has 3in x 3 in hitch) frame made to attach a chest truck box to with a hangar for the steps. (See px). This gets a lot of stuff out of the camper and the back seat. I'm still modifying it to add 5 gallon jerry cans for grey water and diesel.
Cheers....
