Mark,
Using the awnings will not make a difference. Coincidentally we both experienced the same problem, the same week with the same year/model bus. As I recall you we driving north in heavy rain in the same front that I caught driving west to east.
The reason I say the awnings won't make a difference is that my bus is kept in a warehouse at my office. It had been there a month before I left for Talladega. Obviously it was dry. After 2 hours of driving in a heavy downpour both slides started to squeak, occasionally at first, and then with every bump in the road.
It is my opinion water entered little by little through the gasket with the torques and twists on the frame. The wet gaskets rubbed against the clean metal making sounds like a squeegee on glass. Now we have to find out from Prevost if that is common and if not what do we need to do to remedy the problem.