From what I've read, the fresh water tanks (kinda white in color) are made of HDPE which is a high density polyethylene plastic .. the same thing milk jugs are made of.
The repair method is to 'plastic weld' them and glue will NOT stick nor bond on HDPE material.
I did find there is a 'glue' specifically made to repair HDPE tanks. One must 'flame the repair area' with a propane torch, test with water so it 'sheets, not beads' before applying the glue. Fiberglass material may be added to the applied glue for strength.
The glue used was "Mr. Sticky's Poly-Bonder" that seals/bonds Polyethylene, Starboard, PVC, ABS, Hypalon, Lexan, Fiberglass, Wood, Metal, Glass, etc. www.mrstickys.com
is the web site. It is mfg by AAI, Fair Oaks, California.
Yes, it does work and used it to repair the fresh water tank in my XT276. I failed to leave the drain petcock open over winter storage, allowing a little water to freeze and bust the internal threaded 'boss' the petcock screws into.