On the interior it is planned to have a framed bench but rather than interior storage it is wanted for exterior storage with a hatch door that can be pinned open or closed. Somewhat like a tour bus luggage storage.
Would it be sensible to frame this like a low doorway or window? Would it be okay to attach the bench wall side to the header of such an opening?
Residential style doors and windows aren't really designed for pressurized water spray from every direction like what your trailer will encounter so weatherproofing it will be the hard part. I'd keep an eye out for some different implementations on buses, trailers, aftermarket truck beds, etc. Try to see how they handled it, ask the owners how well it works, and then see if that's something you can copy. Structurally I think it looks fine as long as you have enough support at every corner and your header is strong enough.