I would find out if you have a leak before you tear into it.
I would first rinse the tank with the dump valve open. Close the valve and fill with clear water. If it leaks it should not be too nasty this way.
If it leaks then you will need to pull the under bumper wrap, belly skin and tank pan. In that order. Wear eye protection 'cause the sheer volume of stuff in the underbelly will amaze you. The pan should drop straight down once the belly skin is removed. The tank is held in place by the pan. The pan and a styrofoam tray is what hold the tank up, so drain it before removing any bolts.
My experience on the 65 Safari I had was that the bottom of the pan rusts out form water entering through the bumper and the belly skin conceals the rust. If the tank does not leak, I would keep an eye on it, but only open it up if you are prepared to have a new pan custom made. If you need a new tank, I think Inland has them.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato