I misstated the problem. It's not what to use to fix the leak, it's how to get to the tank and remove it. Rexhall puts the tanks gray/black between the rear tires over the axle on WCC chassis. The difficulty is is removing the gray which can only be accessed after removing the black. Removing the black involves the vent, toilet and drain which appear to be doable although not easy. The real problem is with the gray. The shower appears to be glued in placed as well as the drain from kitchen and bath. Cutting isn't a problem reattaching them is. That's where my problem is. Which is why I addressed it to Rexhall owners.
2005 30' Rexhall Vision, W-22 chassis, 19.5 tires, 208'' factory WB, 6000 CCC