I not only keep ours in the closed position for summer use but additionally seal the gaps (quite large) around its edges with duct tape to ensure the best performance out of the OTR A/C. In the winter when that air is not as critical to the performance of the HVAC we open it slightly to allow some fresh air in.
This passage is allowing air to flow in and out of the bus weather the OTR is running or not. When we are stored or parked in the winter I open it all the way and it allows fresh air in and helps keep things from getting stale or musty.
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