Made some good progress last night. Got all the subflooring ripped out, everything but the front and back panels off, and started to look at the floor.
These holes is right above a battery (not sure if it came with the bus originally or was put in later, I'd like to move it if possible). I'm wondering if battery acid played a role.
I hit it with the wire brush and it cleaned up pretty well. Hopefully I'll have the floor treated and painted soon.
Plan now is to: wire brush bad rust -> clean with TSP (is this also a degreaser?) -> treat with Ospho -> paint
Also, I want to take the driver's seat out, but the seat bolts have a weird metal cap right on the bolt. Anyone know anything about these?