This whole situation sounds like bull to me. Even if the water tank were riding on the axle, I doubt it would do much to it. If it did bend it, it would likely spring back once the weight was removed. Someone said it, take it to a reputable shop. I smell a rat.
Trailers are notorious for having poor axle alignment right from the factory. So says the guys who aligned mine at Stanley Springs. Axle hangers are poorly placed and the problem magnifies when you try and locate axles accurately onto the spring hangers. The specs for acceptable axle alignment are fairly tight and most frame builders fall short in execution. This is just one more way the public is being taken for a ride on trailers. If your problem is legit, and I doubt it is, and it somehow falls under warranty, it will likely fall under the frame mfg's warranty and not the trailer builder. Good luck and keep us posted.