Wow ok, so first I used a product called Red Kote. My bikes tank was 4.2 U.S. gallons with baffles. I bought a quart sized can, poured 3/4 can into tank, rock/rolled tank for 10 minutes then drained back into original can (about 1/3rd the amount I poured in came out, also some spillage occured in the process). I waited 8 hours then recoated using the remaining liner fluid. Empty what you can of the liquid then let dry.
***I would recommend giving it a few minutes rocking/ turning the tank after you drain the liner that is still liquid from the tank in order to help disperse any pudding that might happen around the baffles after each coat.
So long as diesel fuel hasnt been in the tank (you would need to sandblast the metal if it was) you can degrease and rinse and when you think its clean and ready to move to next steps immediately after rinsing
You can pour in some acetone to dry up the leftover moisture, drain what you can of the acetone then pour in the Red Kote.
As for the rest I guess thats something you will need to work out cause i never did a tank that large sorry.