I did a tank this fall and used Apple Cider Vinegar. It's 5% acetic acid. I took off the gas cap, lock and petcock and taped the openings with really sticky duct tape. Inside was a gallon of the vinegar and a six foot length of light dog chain. I rotated the tank and shook it like mad every 6 hours for 3 days... voila! The tank looked like new inside. I immediately rinsed well with 1/2 a box of baking soda in a gallon of water, then rinsed with a garden hose for 10 minutes. I heard that it might flash rust so I sprayed the inside with some engine fogger I had in the garage.
It's now painted and on the bike with a new gas cap. I'm not sure if regular white vinegar is as effective, it's also 5% acetic acid, but I'm now a firm believer in Apple Cider Vinegar.
One tip... I learned to shake the tank directly up and down to get the chain and vinegar to the top of the interior of the tank. It's easy to shake it back and forth but inside the top part directly under the gas cap only gets wet and not struck by the chain to knock the rust loose, without this motion.