Clean it, burn it, season it, and cook on it.
The only thing I would check for is to make sure it does not have an epoxy liner. So long as there is no liner, its no different then a fuel oil tank or a diesel tank. Slightly different then a propane tank but not much. Both of the comp rigs I cook on are old diesel fuel tanks.
There are many options to clean it, I used dawn dish soap, you know,,,, the one that cuts grease lol.
Good Luck which ever way you go.