Hey Dave- I had the same issue. I had to re-create my frame the best I could. I completely redesigned the mounting and the pans. My pans were completely shot- both rear gray/ black and the front fresh.
I modeled fresh from the '75 I got the new fresh tank from. It used a drop down steel frame "shelf" with a 1" plywood bottom that slides onto the shelf.
I re-designed the rear to work the same way, but used 2 plywood bottoms, so I could pull them down separately if I ever need to.
The rear cross bar, had a lip on the bottom that extended rearward. This is what the old black/ gray pan bolted to. I felt that this was a major reason for the damage, as it trapped water in the pan. I had a rear cross member fabricated so that the lip was on the forward side of the cross member. I used the lip, along with the skid bars to become the bottom shelf for the plywood that the tanks sit on.
I cut the holes out after mocking up the tanks in place and used a hole saw, which was kind of tough.
I plan on keeping the bumper storage hinge plate AWAY from the rear of the camper so that the water can now simply flow down and out the bottom of the bumper storage with holes in it. I'll use this storage for my dirty sewer hookups, and/or anything else that can get wet.