AFAIK, the bottoms of the tanks have a rounded shape to them when they are built to accomodate the "sag" at the bottom of them. Since the tanks are plastic, the tank bottoms will not be flat because they hold a few hundred lbs of liquid. If you have an insulated underbelly, the material (coroplast) will look like it's bulging more in some spots.
I'm guessing that the sag in ours is about 1-2".
Tank support straps do break once in a while so if it looks like a lot of sag, you may want to investigate further. With an empty tank you could try pushing on the tank bottom to see if it seems loose. Also, occasionally a plumbing leak can happen in a trailer and will cause the insulation and underbelly to sag more so another reason to investigate if it doesn't look right.
There are cross-members that go from the left frame I-beam to the right one. Tank support straps then run in a fore/aft direction between the cross-members. Sometimes the strap welds separate where they attach to the cross-member. In some trailers, travelling with just one tank of full water will cause you to go over the GVWR. Frames are often not built very strong to start with and the weld quality can be questionable which can both eventually lead to a sagging tank(s) if you travel often with liquid in the tanks.
You are probably okay but if you did want to remove the coroplast that's a time consuming job. Some have cut sections out (with 3-sided cuts) just in the problem areas to fix something or investigate and taped the piece back in place.
Gil & Deb & Dougal the Springer Spaniel
2014 KZ Spree 262RKS & Ford F250 supercab V10 4x4 LB