Same problem on my '73. I sealed the heck out of everything back there when we put the new floor in. Luckily (I hope) only the first few inches under the shell for about two feet width are fouled. Still haven't decided how to approach repair, or for that matter if I really want to at this point. May just leave it for now.
As always, great post and lots of good info. Thanks Tom.
A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty
How frustrating I will be installing my floor and black tank soon. Now I'm rethinking the type of materials I will use for the bathroom floor and black tank box. Thanks for sharing your experience, Tom.