For what it's worth, I recently cleaned out a 400F tank that sat for about 5 years with some gas in it. Opening that tank up almost knocked me out! I didn't faint, but it did give me a headache for a day and the foul odor lingered in the garage for a few days. I had heard about how bad old stale gas was, but I never expected it to be this bad. Anyway...
I too first used vinegar, which did a rather goo job of getting out the rust. However, the bottom of the tank showed some tar-like residue, along the seams it was most evident. I put in some ball bearing to help break it up, along with some vinegar, but it didn't seem to help much. I also tried ethanol, which seemed to just soften that gunk up, and then the bearings actually just sank into that molasses-like mess. Not giving up, I finally got some acetone and let it sit in there for a couple days (not filled, just to about an inch above the molasses). This did the trick. All that crud was finally dissolved and I was able to clean it out. Did two rounds of acetone, flushed well with water, then did another round of vinegar. It's now looking rather good. Note that this tank was not previously lined, so I don't know how acetone might work for you but I wanted to at least relay my experience in case it helps.