I would have not expected a lot of rust on a 2011 camper, My 2010 TT has a bit here and there but not a lot.
My pop-up frame rusted after several years so I set it up on jack stands on the drive. Out came the creeper and Rustolium. Not a very comfortable way to paint the underside of the camper.:(
when I purchased my 2010 TT and the frame was one of the first thing I looked at while my DW checked the floor plan. At least I can slide under the TT.
Since the dealer and the manufacturer do not know the units history it is not surpising that they would not do anything about it. I would have thought the dealer might have mentioned it but he probably never look under it.