I'd be concerned about gas vapors. Rather than tinning with solder you could fiberglass. There are some good gas tank patch kits at auto parts stores. My son patch a 60 gallon tank on his motorhome. I see gas tanks from $22.95 to 34 dollars on the internet. Next time you mount the tank cut a piece of rubber, I use old bicycle tire tubes and hold it in place with electrical tape while your mounting.
Good Luck