Thanks for all the replies.
I'd been wondering where the fuel tank access panel might be, so it could be one of these "different metal" patches.
The bus is currently at a mechanic right now, so I'm going based off memory/pictures...
My 2x main batteries are closer to the front of the bus (right next to the entry stairs), not under the rusted panel on the right-hand side. But, an accessory battery for the lift is an interesting thought.
When I checked quickly the other day, I didn't automatically see any wires running from the lift to the main batteries. Do the lifts ever have their own battery?
I didn't get a good look at the left-hand panel as I pulled up the last of the plywood in near-darkness a couple nights ago. If memory serves, it's all the way to the left-hand side of the bus whereas my fuel tank is centered under the bus.
I'd rather not pull the panels since I don't have any welding skills