We had that happen in a previous MH. I bought some 1/2X1/8 sticky-back weather stripping at Home depot and put the sticky side on the frame, starting at the hinge on one side and going all the way down and around to the hinge on the other side. It made the window much tighter when you pushed the locking levers down. It stopped our leak. I planned to put a drip eave over the window like you see over some of the doors on some older rigs but the leak stopped so I never got around to it.
John & Gloria
South West, Florida
2009 Leisure Travel Serenity