Out of curiosity, did you coat the inside of it with IIRC it's called Red Coat gas tank liner? It's a super sticky red epoxy kinda stuff that will keep the tank from rusting, forever. It'll even hold up to bad gas so if the rig ever sits, you can just clean and flush it with new gas versus replacing the rusty tank again. It's not cheap, a quart of it is $35, and you might need two.. but it's cheap insurance.
This stuff..
https://www.ebay.com/itm/134744769523
