To the gasket question, yes. On my bedroom slide there are three weather seals/wipes so to speak. One on each side and one on top. The one on the bottom I believe to be on the inside and can't be seen if there is one. My unit being a 2001 we should be simular. The bottom gasket, that's seen, IMO would seal the slide when the slider is 'IN'.
After inspecting my unit for the Waste drawing, would suspect the air might be coming up from the basement through the openings for the water/drain lines that goes to the sink. As the basement is heated when the furnance is operating, believe these openings are not that tighly sealed. A difference in pressures causes the air to flow into the unit through openings in the frame and basement.
I'm working on a photo post of these opening as we speak as well as the furnance ducting. Will post when completed.
Hope something helped.
2000 Alpenlite, 29 ft Valhalla - 2014 F350