Got the tank back. He charged $100 to reinforce the two lower mounting studs and reweld the lower seams, then pressure test.
The area that was reinforced is right at the lower mounting studs
The original build just had simple nylon washers on each of the four studs, not much support and might have helped to crack the seam.
Here is the trailer's frame that the tank bolted down to. This area is between the main tub and the nose box
So I decided to replace the washers with some dense rubber sheet of the same thickness as the nylon washers
Some material on all three frame sections to help mount things a bit better.
So the tank is now back in, the nose box bolted down, the batteries reconnected and the water tank filled.