What about a panel made of the same composite wall panel cut to fit the opening? Obviously the interior layout of the camper will determine if it is possible to install and remove the panel. It would suck to install and remove, but if it is only done for seasonal changes it should not be too bad.
Sorry to be a pest, my job gets crazy busy in the summer, so I have recently come to the conclusion that the best time for me to go camping is in the winter. Living in the frozen north of plant Hoth means that literally every off the shelf option for a camper is no good. It also rearranges priorities vs most builds as things like solar are essentially useless and redundant heat sources are at the top of the list.