I built storage for my sewer hoses (Lippert Waste Master) from square aluminum tubing. It's mounted at the back above the bumper. Some place like Metal Supermarket can cut to length and a non-ferrous blade in your skil saw or miter saw can make other cuts for you.
Brackets on the bottom bolt to the frame rails.

End piece is 6"X6" square tub turned sideways and cut in two pieces for door. Piano hinge riveted to pieces with catches to shut. This piece and the end cap are the only welds required. Brackets to the frame and to secure the tubes together are bolted on. (Picture taken before second tube added and hole cut in end piece.)

Pull out the main tube and you have access to the extension tube.

The fitting to attach the extension is too large for main tube (4"X4") so I built a box from a piece of 6"X6" square tube.