Is it a fiber glass or a plastic tank? Fiberglass is no problem to repair. Just need to sand it back to clean material and reglass that area.
Plastic might be a problem. ABS and some other plastics do not glue very well. There are tricks including flame treating the surface to burn off impurities so it will bond with the adheasive.
What your is the coach? Have you been to www.inlandrv.com
Andy has some tanks available for older units and might have one for yours.