Unfortunately keeping the tank 100% dry is IMO an impossible process. I replaced my circular access hatches with circular hatch plates that are made of clear plastic. You can see through them. The plates are almost always covered in dripping condensation. The fuel tank, typically being filled with liquid that doesn’t change temperature as rapidly as the surrounding atmosphere, being mostly sealed up in that compartment, which by nature of location is moist, creates a very effective condensation generator. Short of installing some sort of ventilation system, the fuel tank is going to be moist/wet frequently.
Think about what happens in the Summer. Let’s say you’re living in Southern New England in July/August or how about Florida or North Carolina, It’s not uncommon to have very humid and warm conditions for weeks on end. Your boat is out in the sun roasting and then you fill up with a 100 gallons of gasoline that is likely to be significantly cooler then the surrounding air and store that gasoline in what amounts to a giant Aluminum beer can located in a confined space with almost 0 ventilation.
Think that situation might generate a little moisture?
I think the fuel tank is going to be wet/damp a lot of the time no matter what we do. I think keeping out the salt water from leaking in is the best most of us can hope for.