Had the same issue.
Got to a CG and son noticed the underbelly hanging on one side.
It had worked itself off the frame 'shelf' and dropped. There wasn't a single fastener installed anywhere....guess they just jam the material to one side of the frame, bend it until it pops onto the opposite side and call it good :(
Whilst under the trailer to repair it I noticed that two of the nuts from the straps that secure the water tank onto their 'ready-rod' supports had completely fallen off....hence a bit of a saggy tank.
Re-secured the water tank and then added self-tapping screws (pre-drilled the beam) and large washers wherever I could find a cross-member to keep the underbelly up.
Even with that, there were spots where the underbelly appeared to be sagging, but it was just for it to get under the holding tanks.