[13Echo70271]

Hi Blue,
The fill vent could be a problem, but if you have checked it already,
Does one or both of your round inspection ports/hatches access the fuel sending unit, fill hose and vent hose connections to the tank? In destroyers pics they are all on one end of the tank. My 88 has a plastic tank and the sending unit and fill hoses are at opposite ends of the tank.
I think if I could access them, I would check those fill hoses for rubber crud inside,
Tipp the boat so the sending unit is at the low end, drain the tank, using the boats fuel line or a siphon hose. If you can, use up most of the fuel first.
remove the sending unit and using the sending units opening in the tank inspect and remove any debris. Any debris left could be swabbed out with a small rag tied onto a stick through the sending unit opening. You may find you don't have to remove the tank.
Do NOT use a shop vac to remove debris, as fumes drawn into the vac can ignite(ask me how I know).
If you have to open it up bigger, I would consider cutting two 14" x 14" or so, openings around the current openings for a couple aftermarket floor hatches (ebay). That will give you more working room. Much easier, cheaper, faster with fishing season upon us.
Best of luck,
Tom (13Echo70271)