Straps are probably welded to the frame, the tanks are then installed when the frame is turned right side up before the floor is laid in place. Your best bet is probably going to be to cut the straps off, drop the tank. Being careful not to damage the line to the pump. Once it is on the ground your best bet is going to be to take it out, measure it up and go searching the internet for a replacement. If your RV is new enough you might contact the manufacturer and try to find out who make the tanks for them. Call the supplier directly and order a new one. Those things are not easy to repair. Once you have the new tank, go buy some new strapping, cut to length and bolt them back in place. It will require drilling some holes, but that is the easy solution.
Don and Lorri
2007 Dodge 3500 dually