The ladder is totally supported by the box. It has a cross member inside of the box terminating the flanges. Lots more support than the 90 degree bend and flanges holding it by 8 screws on the fiberglass panel by the OEM. housing a spare 20 lb propane bottle probably doesn’t carry any more or less risk that the 2 30 lb bottles up front in the fiberglass storage compartments. The boxed steel construction of the rear box is really strong and most likely has a bit more integrity than the fiberglass ones up front. Perhaps not as exposed to hazard as a side impact but definitely stronger.