I have never found a glue or epoxy that will bond any density of polyethylene to ABS. You might get something to hold for awhile, it will eventually fail.
You need to find out if the tank is Poly or ABS. Try a little
ABS pipe cement somewhere on the tank. If it starts to melt the tank ,its ABS. Just because its black means nothing.
Valterra makes ABS fresh water tanks that are black. The logic is algae can not grow in them due to no light being able to penetrate the tank. The bungs however are cast into the tank and then tapped.
Fresh Water Tanks Archives - Valterra.com | Valterra.com
When you start talking buying plastic welders or spin fitting collets, you're already in new tank price territory if you play your cards right.