Charlie, can you physically remove the tank from the boat?
This makes the work MUCH simpler, and it means you can use a hose and detergent to clean out the tank, as well as the aluminum
bits that may fall in while you are cutting the access ports.
I'm not sure about the shop vac, I suspect ignition danger
(from the diesel fumes through the motor's brushes/sparks) if nothing else. We used Liquid Tide, which even dissolves Cosmoline, and warm water
because we didn't have access to hot water
. The nylon brushes
are fine. If there are baffles in the tank--you may not be able to work around them. Removing the tank from the boat means at least you can roll/tumble the tank to get some action all the way around in it, repeat and rinse until clean. Then we gave it an alcohol wash (to help the water evaporate) and left it in the sun to thoroughly dry.
Would have taken it to a shop for steam cleaning
with a "steam genny" but that would have meant a long trip and high price--the waste is hazmat. Working at the boatyard, it just went in the "used oil
waste" tank. If you can rent a steam genny or hot water pressure washer, that's a good tool for the job.