So, I have had good luck with evaporust, followed by baking soda and water, followed by a quart of two stroke. Evaporust, I believe, is considered non-haz., so not very chemical-y and won't damage paint.
I have tried vinegar and run into flash rusting issues, not so much of an issue with evaporust.
After cleaning several tanks I have concluded that there is no way around it being a messy job with some sort of waste.