That stinks. As you can see there's no great way to get to it. I think about this stuff all the time as I put mine back together from a full monte... In this case, you could:
a.) drill out a bunch of rivets underneath, with the hopes of being able to pull the belly skin back enough to reach behind... OR
b.) Cut a nice clean access hole, or square. And then cut a patch to cover it. You can make the patch look nice and even use screws for an access panel.
I'd probably do option "b". Any way you look at this, you'll need to be able to get a wrench on the back side. So pick your poison