If you can come up with those pics, I'm sure we can find a solution.
If it's the same inspection port as my potable water tank and holding tanks are (were) the only thing in the universe that will stick to poly, bend and flex without losing the adhesion is Permatex #2 :
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The Hunter factory used 4200 or 5200 on the outer ring and it wasn't even in contact with the poly when the screws were removed.
Of course you'd be a complete fool to believe what I say on some far flung boating site so buy some (a few bucks) and smear a blob on the exterior of your poly tank and see what luck you have in trying to remove it after a day or so. Impossible. It never hardens. The only thing that removes Permatex #2 is any form of alcohol. Use Permatex to seal the outer ring to the tank, not the "O" ring under the cap.
As far as the large "O" ring in the port goes, let's see what shape it's in when you remove it.