I just followed that... and found this quote...
"They originally used coated plywood, the switched to PVC, and most recently have switched to Fiberglass Sheeting of some sort."

Well, to tell the truth, the coated wood probably lasted a good 30 years, and I bet the PVC will last at least 10, if not more... and is a lot less hassle than dealing with coating some plywood... so I will likely go the PVC route anyway.

I saw this with regard to using PVC to replace a holding tank top...

"Making the new top was dead simple. I c-clamped the old plywood top to the PVC, drew a line around it with a marker, removed the old top, cut the plastic on the outside of the line with a jig saw, re-clamped the two tops together, used a router with a following bit to trim the PVC top to the exact size of the wooden one, and with a drill press drilled the new screw holes through the old ones to insure they would be both straight and in the right places. After again separating the two tops, I rounded off the bottom edge of the new top with the router. The hardest parts were moving the c-clamps around as I worked and doing a little freehand router work in the areas where the plywood had been damaged. I also sawed out a hole in the top for the inspection plate and drilled holes for the bolts to hold it in the PVC top. I roughed up the edges of the PVC where caulking would go with 80 grit sandpaper to get better adhesion."

Frankly, I would only replace an entire holding tank... if at all possible. I am only dealing with a fresh water tank.

The seal seems to be the biggest issue... but folks are suggesting the red silicon seal... "GE 100% Silicone Sealant for Plastic Sheets and other Plastic," and seem to be happy with the results.

Thanks to you again for some insight on this and the suggestion to check on sailnet.