RV holding tanks are typically ABS or low density polyethylene (LDPE). ABS is easy to work with and easy to glue. Patching or gluing polyethylene is not easy and is best left to a pro. - that is, *IF* the LDPE damage can be repaired. If the tank is ABS, depending on the location and size of the rupture, you *might* be able to get some sheet ABS and glue a patch on. Patching may not last though, esp. if LDPE.
Depending on how bad/big the rupture is and the type of plastic, you can buy patch kits like this one:
PPL Motorhomes You might be able to get some sheet ABS from a plastics shop and make a patch to suit the actual sixe of the damage.
If you google "RV holding tank repair", you will find lots of info. on repairing tanks such as this one:
Everything about RVing.New tanks aren't all that expensive and are in the $200 -$400 range. They come in a large assortment of shapes and sizes and you should be able to come up with one that will fit. If the existing tank is ABS, I would stay with that because the inlet and outlet fittings can be glued on and less likely to leak. Using an LDPE tank would require using mechanically attached fittings. For all the effort and cost of trying to deal with the existing tank and piping, it might be easier to just replace the tank. I see that there are a lot of vendors at the moment that have tanks on sale.
One source of ABS tanks:
ICONdirect.comOne source of polyethylene tanks:
plastic-mart.com