A twelve years old Scamp
window frame could easily be full of black rotting muck that prevents the weep slots from doing what they are designed to do. The best is to slide the window open part way and then pull/push and lift out the rubber channel that the glass slides in. The glass then drops down a fraction of an inch and the top edge will clear the upper frame. Lift it out and clean up the frame and the rubber channel thoroughly. Pay attention to how it goes together, re-assemble in reverse order.
I hose my Scamp
down once a year whether it needs it or not and every time I wash the channels with a hard stream of water into one weep slot and watch the dirt wash out the other. (As long as the trailer is level the weep slots drain well, without any water getting inside.)