There are several manufactures known that they did not mount the water tanks very well.
If you need a new tank, try this website.
Tank-Depot.com At least check out the site, so if the RV repair place says your tank is something like $850, you can say a replacement tank should not be that much, a tank at this site should be in the $200 - $400 range, depending on it's size.
One guy posted he had to reinforce his tank by using some angle iron and flat stock steel, under the tank.
What I did when I installed a new pair of batteries in my RV is put some unistrut under the storage compartment, with a bolt to the frame of my sidewall, and on the other side a L bracket and some all thread to keep the unistrut tight against the bottom of the storage compartment, to carry the extra 70 pounds of each battery.
Fred.