Sleeper modification questions

No idea. PO sealed it off to convert the rear sleeper into a shower.

Oldtimers recalled everything from refrigerator accordion strip to bicycle innertubes sealing these sorts of setups. There was a lip above it on either side, which would logically keep rain out at a stop, but there was nearly a 2" gap with the cab down. There was no suspension on either cab, so they didn't move around much, but I suspect there would be enough frame flex to cause some chaffing of any gasket material used regardless. Another major concern for the gasket is the heat, since the sealed opening was directly above the turbo, and I'd burned myself on the aluminum body sheet in the sleeper before I insulated it.