I got my idea from another thread where someone did this. They used steel. I used aluminum plate. I painted the top & bottom white to kind of match the camper.
I used Rotopax mounts & containers. I can carry water jugs (2 gallons = 16 pounds) or fuel, 3.5 gallons (19 pounds)
One jack weighs 20 pounds and I figure the bracket can at least support 1/4 the weight of the camper.