I noticed last spring our rails/support were fastened with self tapping screws. Many of the screws had broken and the rails were bowed... some of the crossmembers were held in by only the weight of the tank!
Before ours was dropped, I removed all the self tapping screws, drilled holes and installed 3/8" galvanized bolts to hold the supports/rails together. Some locations were a bit tough to get into, but in the end all were replaced. This seems to have taken care of our problems.
We don't usually travel with fresh water on board. When we do, I keep an eye on it now.
Happy Camping!
John.