That load spread out over that area on the roof I do t see any chance of damaging the roof. But I would NOT want to have that weight up there.
A bladder is the right idea, but on the floor somewhere. IMO
Look at wake board boat ballast bags. Can get em in many shapes and sizes and can get them made to any shape and size you want as well. They’re tough. And close out model stuff is super cheap if the size works.
Anything from a couple hundred lbs to 1100lb sacs are readily available.
That and a ballast pump pre made cig lighter plug in with a long cord and hose.
I’d search for a spot inside the camper that you could stash that thing, personally.
Don’t do it often, but have packed 50gal in one of my ballast bags on the back seat floor of the truck, to camp and then unload it once out tank is empty.
2016 Ram 2500, MotorOps.ca EFIlive tuned, 5” turbo back, 6" lift on 37s
2017 Heartland Torque T29 - Sold.
Couple of Arctic Fox TCs - Sold