We have a 2007 Keystone with the front slider window to the truck cab window. We took a quick 3 week trip to Alaska from Washington this summer and after driving 6+ hours in a rain storm we also had a water leak at the front window. I duct taped all around the window's slider joints and center joint and across the top of the window hoping this would temp fix it, it didn't. The front parts of the cushion over the water tank and the first 6" of the couch/bed cushion were soaked. The rain abated and the problem stopped until we were coming back down from Dawson City Yukon in the rain and again the cushions were soaked. I noticed the windows had two slots one under each side that went through the frames in the bottom of the window. I duct tapped those too and problem was fixed. in the next 3 days we drove through a LOT of rain and nothing leaked. When we got into cell phone coverage I emailed FWC's service guys and they said these were "weep holes" and that plugs were available for them. I guess my question is what are they weeping for? The side window doesn't have ant of these weep holes and neither does the one over the cassette toilet. I figured out the leak finally when upon close inspection I saw fine grit (talc like) in the window channels matching the same color as the gravel chip seal roads we'd been driving on in the rain.
I understand condensation and we always have the roof vent open some and at least two of the side windows venting when we sleep or use the stove or the heater. This worked great in Alaska this year and with the exception of drying out the cushions we never had a condensation problem. I'm either going to leave the duct tape over these weep holes of I'm going to seal them with silicone, it just doesn't make good engineering sense to put two slots in a relative high pressure area open to road spray and then expect that nothing will penetrate these two slot drains.
BB