Oh my god this worked so well.
First I rinsed the tank thoroughly with water. Then, and this is what worked better than anything else I've ever tried, I dumped in a box of 1500 BBs (the small brass balls typically fired from a BB gun) and shook the sh!t out of it for about 20 minutes.
Then I dumped out, and saved the BBs for re-use (they rinsed perfectly clean). Rinced the tank with water again and then poured in a gelled rust remover. This is a much thicker version of the same stuff I used to use so it clung to the inside walls of the tank better. I shook the tank with this stuff in it for another 20 minutes, poured it out and rinced again with water.
Finally I flushed the tank with some fuel mix and then a spray of WD-40. I also plan to add some water remover to my first full tank.
The tank looks so clean. The BBs did an amaxing job.