That window is designed to be installed in either direction (flipped on the horizontal plane), so you can have the slider move L-R or R-L to suit your application.
Because of this it has condensation weep drains on the top and bottom rail. Likely, the rubber inserts for the top drains are loose. After window is installed, remove top rail rubber plugs, add a quality sealant to the back of the plugs, re-install plugs, clean excess. Done.
Mine had the top plugs, but they slid in the openings, and hard rain/washing caused mystery leaks, until I solved the mystery. (Silly me).
Remove plugs from the bottom rail, so that condensation can drain to the outside.
Oh, and if he hasn't been over to this site yet, send him there. Lots of good information, and helpful folks.
Cargo Trailer Conversions
1995 Ford E-150 Club Wagon Chateau Van
2012 Cargo Trailer Conversion Camper/Hauler