A 5 gal. jerry can full will weigh approximately 40 lbs plus the weight of the holder.. This is enough to tear the jack bracket off the camper after hitting a few ruts or pot holes. You may want to consider through bolts (4) in the rear frame members of your camper to which you can attach a can holder which can be removed when not needed. You may want to look at "rotopax" and their mounting system for ideas.
I have bolted a 1/4" aluminium mounting plate to the underside of the camper to support the weight of the fuel can and holder. The outside edge of the holder is bolted to the jack bracket to help stabilize the load. The support plate is 30" x 9" and is mounted such that 8" sticks out to the rear of the camper the can holder is 7" wide and sits on the outside 7" of the plate (leaving a 1" gap between the back wall of the camper and the fuel can holder. 2 1" x 1" aluminum angles are bolted together and then bolted to both the corner jack bracket and the can holder to stabilize the load. I have been rear ended and the bottom of the holder is about 4'9" off the road, so no burst fuel tank!! (only the truck need repair, not the camper)
If I were to build it again, I would try to find the frame members and mount (through bolted as high as possible) a large plate to the back wall (with a 1" gap), then mount the can holder to the plate. The can holder could then be removed when not needed.
good luck with whatever to try to do.