Hopefully, nothing is wrong with the black tank. You're best just to let nature and time thaw it out, then dump it. Once it thaws, you'll know if there is a leak or not. If the tank was not completely filled ... like up into the toilet bowl full, there may be enough air space, that the freezing expansion had somewhere to go. It pushed up instead of out, and (hopefully) did not damage the tank or drain pipes.
About your water heater. If it's ruptured, you're best to replace it. Check with any RV dealership, Camping World, or any place that stock RV parts, and you'll probably be able to find a replacement. Will it be easy? Probably not, if you just use your head, and think it through and disassemble only what is needed to reach the tank for a replacement. It's very do-able.