How should I address this window leak?

Resealing the window BEFORE exploring other possibilities could be very frustrating as I've gone thru the same backward steps and will share my success only after having wasted efforts that had no success. My leak in a sliding window as well as fix windows revolved around the channel that the weep hole is part of. That weep hole allows the water that flows down the glass and diverted away from the glass by the rubber strip. Some of the water by gets behind the rubber cosmetic strip to collecrt in the channel that the weep hole is part of. Thru the years the channel will accumulate fine pollen like debris that sticks to the glass and washes down with the rain water. This debris will restrict the outflow of water even if the actual weep hole is clear. The channel needs to be cleand out, much like home rain gutters. The gunk in the channel will restrict the free flow of water to the weep hole and spill over the upper edge of the channel on the motor home interior side causing the type of leak you described. Pulling up the cosmetic strip will expose the channel for cleaning or for that matter to inspect to see if it does need cleaning. The cosmectic strip is very very easy to remove and replace using the same strip.