One used to be able to buy replacement tanks for RV water heaters, I have replaced a couple, years ago. If the rest of the water heater is in good shape, and you are pretty handy, that is one option. It is probably one of the older type, that has a pilot light, and those are very handy, as you can just leave it on 'pilot' and still have hot water, when you are boondocking, and not use up a bunch of propane ! You can't do that with the newer electric ignition ones.
As for your black water tank, our local RV store, www.camperland.net
has some expensive, very thick, wide, aluminum-color tape, that they swear by, for fixing those ABS & PVC tanks. Our RV store sells it by length, or a kit is $34. It is kind of like the weather dam stuff that roofers use. For years, people fought with their cracked tanks , and tried fusing, glueing, and all kinds of remedies for them. This is supposed to work.