I just pulled my belly covering down to take a look at this last week. I used a 2 inch square tube with plates that sit on top of the lower frame rail. I saw the design on another post here. It runs from side to side and sits on the frame at the rear spring perch location. After looking at my tank, it was sagging down even when empty (3-4 inches below the side frame rails). I never filled it before installing the brace so I don't know how far down it would go.
The tank is supported by channel on the front and back (side to side) and has two sheet metal straps that run between the channels from front to back on the tank. The straps are held in place with pins, so they can be removed. I really don't know how likely the tank would be to fall out, I think it would only sag. After filling it with the brace installed, it sure didn't seem as though it took 60+ gallons. I wonder if some of that sag is part of the design to give full tank capacity. In any case, I feel better with the new brace installed, for now.
I also installed a valve on the 3 tanks vents that hang down in the rear. I opened them during the fill, and closed them when water ran out from them and the vent next to the fill opening in the docking station. I believe my trailer was built in December 2005.