Well I don’t have the correct vacuum diagram for my coach or yours, but I am attaching a diagram that “may” provide some similarities to what we both actually have. If so, then your air flow problem from the instrument panel air register on the driver side should behave the same for max a/c; normal a/c and vent vacuum switch positions?
I “wonder” if it is the vacuum motor pulling the panel door to the full on vacuum position, but then slowly releasing it partially (maybe a leaking pin hole in a vacuum motor diaphragm ?). The fact the air flow is good on the driver side for a short while, then slowly shuts off would suggest a vacuum motor (assuming the vacuum switch control is good with no vacuum leaks). If the instrument panel air registers "air flow panel door" was jamming part way when vacuum is applied, then I would think the air to the driver side would never be full on initially.......but of course that is a WAG.