Some thoughts.
Check with the builder before you do anything. Unless you bought a
one-off, there may be a generic problem and a generic solution. You
won't know unless you ask.
It's human nature to assume the worst, but a lot of leaks are simple
things we've overlooked. Are you sure there's no other mechanical
fitting that could have become loose--cleanout, drain plug, sensor or gauge?
No matter how well you sanitize the bilge and tank, keep ventilation
running at least an hour before you cut or flood with CO2. 5 or 10#s of
dry ice between the bilge and tank ought to do it.
You can rent an explosive atmosphere tester at a tool rental store.
If you have to cut through the deck to get to the tank, consider adding
a permanent access. Lightening never strikes twice in the same
spot...or does it? Same goes for the tank. If practicable, leave a
manhole in the top of the tank under the accessway. A low flanged
collar with thick fuel-friendly gasket and bolt-on cover is the way to
go. With the collar flange up, nuts and bolts are always on the outside
of the tank. Don't goof around with saddle patches and self-tapping
screws, you'll just have to come back next year and do it right.
And most important, if you don't have the expertise and equipment for
welding aluminum, have a qualified shop do the work, you'll save money
in the end. Good luck.