I see what your chatting about. I thought it was on the bottom of the roof, which is the stirrup hold down, but your dealing with the underside of the bottom box. There are rivot nuts that can be installed into the bottom. Like an expandable nut or sleeve that is threaded for a machine nut. Not a lag screw that essentially digs or carves itself into the wood or metal and over time can pull out. As you mention, wood rot is probably the culprit there due to the underside is so close to the road surface and rain, wet roads can penetrate over time. Does appear that there is some staining with a wet area. I don't know I would open up that can of worms on what's under the bottom panel or belly cover. I've offered a link for rivot nuts.
https://www.fastenal.com/products/fa...%20Nuts%22%7C~