John:
On the black and grey tanks. On ours, the black and grey tank flanges are captured the whole length by cross braces for the trailer frame. One one side it is captured by the big permanent cross brace, on the other by a shorter removable cross brace. So it has support the whole length on both sides, but not underneath.
If I understand correctly yours only has a few brackets supporting the tank. I'd be suspicious as well if mine was that way.
On the fresh tank it has straps across since it has no flange. I added a bottom support of 3/4" plywood under the whole fresh tank for support.
We've traveled about 25K miles in ours, virtually always with a full fresh tank. Since we dry camp a lot, I've gone significant distances with the grey and black full before dumping many a time.
And since I have a dump station at home, even when the CG has a dump station, I most often drive home and dump at home if the distance is less than a few hundred miles.
Grey and black tanks are each 32 gallons.