The camper is already higher than you would like it to be; cut the wheel wells, avoid the dreaded blow over. She will tow nicer if you keep the weight lower.
If it were me I would make the fender wells from 1/2 ply (max) and line the under side with thin alum flashing, then seal with your choice, black goop.
I suppose you could build them up out of foam, but then they would need some protection from foot traffic.
The barrel sections sound like a good idea at first because they are rugged and one piece molded, but then you have to deal with attaching the irregular shape and if you want the inside to be flat you'd have to box them in anyway. $0.02
Are you done yet?
