Re: So, about that welding near the gas tank...
Welding near a diesel tank , not much more dangerous than welding near a blackwater tank. If the tank will get warmer than you can touch, you might reconsider...or shield it.
Welding around a gas tank, hot to the touch, NO! Gasoline makes highly flammable vapors and boils easily, diesel doesnt. Shielding a gas tank may not work, vapors have a way of moving around.
Siphon or drain, or just delay your work until the tank is almost empty and remove it.