A "little" belly sag in spots is normal assuming your referring to the underbelly cover material..., but check to make sure that none of the tanks (fresh, grey, etc.) didn't drop from it's support straps, and/or acquire a leak.
If required, you can use a couple self tapping screws to shore the black underbelly material (Coroplast) up but insure your screwing into the TT's metal frame members thus avoiding electrical, plumbing, tanks, etc..
2016 GM 2500HD 6.0L/4.10
2018 Jay Flight 24RBS
2002 GM 2500HD 6.0L/4:10 (retired)
2005 Jayco Eagle 278FBS (retired)
1999 Jayco Eagle 246FB (retired)
Reese HP Dual Cam (integrated sway control)