Check on both sides, especially behind the gas tank.
There is a "cavity" that runs forward if the bulkhead, above the lower sills. This area can collect fine sand and dust, which can be a rust-prone area. I vacuumed mine out very well, and put a bunch of bedliner-type stuff in there.
It probably doesn't help if corrosion is there already, but you will never have better access than now.....