As to the galley trap drying out and allowing sewer gasses to enter the unit, fill the trap with RV antifreeze - I don't think it will evaporate in the time frame you are talking about. You could also put a cutoff in the drain hose.
For me, the easy way to do it would be to just block the drains with a stopper or a rag and let the tank vents do their job.
Happy Camping! ///// Richard D.
2006 4x4 Ford 250 SD / 2007 Flagstaff 827 FLS
One very patient wife and one furry child who travels with us. Forty-two years of trailering and camping, and I still have a blast.