Often the glass is held in place and sealed to the metal with a sort of caulking or adhesive, probably a urethane of some sort. If that's the case, the rubber "seal" on the outside is more for trim and to keep things in position than to actually seal against water. The solution to a leak here would be to remove the glass and reinstall it, something that I would be hesitant to do myself.
A strip of some appropriate tape, such as Gorilla Tape, applied between the glass and the frame on the outside will cut down on the amount of water getting in but will not completely solve the problem.
You could ask at a local glass/windshield sort of place, but it may well be cheaper at an RV repair center, if only because they already have the scaffolding etc. ready to go.