Congrats on the new camper. I can hear you when it comes to the water tank sagging, had the same issue. Took it to dealer, they put on a new tank and looked great until I filled it up with water, sagged big time. I called the factory and they said because of weight issues, trying to make everything lighter, the tanks are not as thick as they used to be in the wall thickness.
After looking at differant ways that I thought would help relieve the sagging, I came up with measuring a piece of exterior treated 1/2 inch plywood 2 inches wider and longer than the water tank. Used a floor jack to push the plywood up tight against the bottom of the tank to flatten out sag and used three pieces of plumbers strapping with 3/4 inch self tapping screws across the the ends and center of the tank screwed into the frame that holds the tank in place. After installing, removed jack and filled tank with water, no sagging. Drained the tank and darn near got it all out except where drain valve meets water level, tilted the camper on the drain valve side and got most all the water out. This might be worth a try, cost me less than $20.00 and about an hour of my time. You could take it to dealer and see if they would do it under warrenty. Good Luck.
2016 Forest River Surveyor 201RBS
2014 Chevy Silverado 1500 4.3L