Greetings and I wish you weren't dealing with leaking diesel! I looked at your other photo you have showing the fuel tank layout
Trawler Forum - ak-guy's Album: Allweather - Picture
Its certainly an oddity to have the tank top part of the floor like that!
I'm sure you've seen the drawings on the allweather website - (it looks like your boat is there?)
Allweather Boats History
That drawing is pretty clear, it shows the wood stringers, radius filler, stringer tabbing, fiberglass layer and the dang sheet of steel on the top!
Assuming yours was built according to that plan, it doesn't show fasteners, but it also doesn't show any floor detail. From your previous photo in your album, it looks like there is some sort of a furring strip around the top edge of that steel plate, I'm wondering if you have some mechanical fasteners under there, or if there was previous leakage and a PO oversealed it with that furring strip. You might reach out to other allweather owners to see if you can find someone else who has dealt with a leak, or at least compare floors to get a better idea of what you're dealing with.
Based on your thought its a steel plate on the stringers, and the construction drawing from the allweather site, I'd say you're looking at draining your tank, removing that plate, and recreating your tank top from scratch - just to make sure you have a bullet proof and safe tank.
If it were my boat, I'd look at a solution that has a separate floor above the tank top, my intuition says don't use a sheet of 10g steel as a floor!
Hopefully some fuel tank gurus stop by and give some more authoritative advice on that particular tank design. Its odd that it doesn't seem bulletproof when the rest of the construction of the boat seems aimed at just that!