If I were you I'd take pictures of the roof and the ceiling with and without water on it. I'd also get a straight edge and check the sag inside and out and take pictures of that. Also take a pic or two of the straightedge on a flat surface. Better thing to use is a string and gauge blocks and measure the sag. Your dealer may well tell you it's normal. If they do I'd demand a letter stating that the amount of sag you have is normal from Forest River and from the dealer. Be persistant about either getting it fixed or getting the letters signed by someone of authority at FR and the dealer. Make at least three copies and send one to Forest River, one to the dealer and keep one in a safe place for future reference. Sign and date the copies and it's not a bad idea to put the VIN on them either.
Now to the fix. They can take the roof off and place a positive bowed stiffener(s) (bowed in the up direction) along side the rafter(s), jack the roof up and secure the stiffener(s) to the rafter(s) and bring the sag out that way. They'll probably want the unit back at the factory to do it.
P.S. Never admit to walking on the roof. Don't let the dealer try to put anything under the ceiling to fix the problem. That'll ruin any resale value it may have in the future.
I'm not trying to scare you. This can be fixed and as far a that goes it's not as terrible as it sounds. It may indeed be a normal amount of sag and may not be a problem at all but, a little CYA goes a long way if something happens down the road.
Salem 29RKSS Pushing a 2500 Mega-Cab CTD!
Gotta go campin!