98taco3, if you tried everything else, and you have a well vented back yard, you may want to try this, it has worked well for me on several fuel tanks gas and diesel, which sat for 30+ years in the desert heat. Take out the sending unit and float assembly, find a metal plug for the drain plug, and other openings below fluid level, go to Home depot near you, look in the paint prep section, for a gallon type can of concrete prep/etch product, which contains Phosphoric acid, ask the sales person in there, or read the label on the can. You want the one with the Highest % Phosphoric acid as content. Do not tell them what you want to do, as they will not sell it to, you if they know. For a 20 + - gal tank 2 gal of this stuff will be plenty. After you buy the 2 gal cans, got to a food store near you, and get 6-10 gallons of distilled water, not purified, or filtered but distilled. After you plug all opening in the tank, set it on a level surface in the back your or someplace vented where kids and pets will not have access to it. Wear your safety face shield and rubber chem resistant gloves. Pour the entire 1 gal of concrete etcher / prep in to the tank, add slowly 1 gal of hot distilled water to it, using a funnel. Let is sit overnight. Later, check on the progress, this solution will dissolve all non metallic organic deposits, rust, warnish, tar, other unidentifiable deposits on the bottom of the tank, and will not eat the metal of the tank. Gently slosh the solution around, to see if the deposits have loosened up or not, the worse it smells and fizzes, the more it is reacting with the gunk on the bottom. You can use a long screwdriver, or a long metal rod to poke the gunk on the bottom, to see if it loosened. Depending on the thickness of you gunk in the tank, you may have to try several times, do not let it sit at any one time longer than 3 days/nights, dump it out, and wash out with yard hose water, try again as before. Please wear rubber gloves and face shield, since you are also made from an organic non metallic material, as far as this stuff is concerned...
I had not had to do this more than 3 times to a very gunked up fuel tank, with 2.5 " of tar looking deposits on the bottom of a 50 gal tank, after final water rinse it was shiny metal, like new, any place the solution reached the gunk. I advise not to slosh it around too much, as it can splash or leak out, and it is unpleasant when it does. A less concentrated solution can be used to wash/soak small gunked up parts in a stainless steel bucket or bowl, but same safety cautions need to be observed, same basic solution.
Hope this helps.
ALFA2
