John:
I have not seen many water tanks fail in 20 years, but I guess it could happen. The tanks as you well know are under the floors or bunks, and that requires cutting out the floors. It has been done by owners and there are yards that will do it as well. It is not cheap to get a tank replaced. Black Tank (the sewer tank) might cost $6,000-$10,000 USD to replace. The tank might cost about $500-$750 it is all labor to cut it out. That is the job. The older 2 digit IPs the tanks are a little easier to remove. But the 3 digit tanks take a lot of cutting to get them out.
So, eventually you need to plan for a sewer tank, black water tank replacement.
I replaced mine with a fiberglass tank now.
Hayden
