If the leak showed up after opening the valve there is about a 99% certainty that the valve is the problem. Even if it proves to be the tank structure itself many times they can be repaired by an "S" code (pressure vessel) welding shop.
Personally, if I was going to the trouble of replacing the use valve, I would replace all of the screwed in fittings at the same time as well as sandblasting the tank and refinishing it. The big propane companies rarely work on jobs like this where the smaller independent dealers seem to be more willing to tackle something like this. If you do any of the refitting yourself make sure the pipe dope used on the fittings is LP compatible. All of them aren't. I've always used teflon tape and pipe dope, sorta like a belt and suspenders, and never had a leak.