Re: That Darn PO!!
Tom, there are compression fittings that are designed to be installed in plastic tanks. No glue, no welding, no threading needed.
I think the link that Jimscal provided is for that type of fitting, we used to use them on chemical storage tanks and they are 100% reliable.
If you think of a plastic miushroom, with a hole running up the center of the stem and through the head? The head of the mushroom has a gasket (often a large flat gasket) under it. You put the mushroom inside the tank, pushing the "stem" through the hole in the tank wall. Then a similar gasket, washer, nut, are screwed onto the part of the 'stem" that is outside of the tank. Snug down the nut, and the tank wall is captured between the two gaskets and there's a 100% seal. Now just put your hose over the end of the stem, clamp it on, you're in business.
No need to pull the tank, just put the new fitting in the old hole, drill or ream the hole as necessary to get the sizes to match.
This is pretty much exactly the same as installing a through-hull in the hull of your boat, if you can't find plastic tank fittings, look for a small marelon through-hull and see what you can adapt from there. The tank kits will have gasket sets, etc, the through-hull will require a little more parts shopping, either way, no big deal, no need to pull the tank!