If you have the ceiling vents open and vent fan going as you drive down the road... whatever problems you have with smells coming up from empty P traps or tank vents are going to be compounded.
Make sure you have water in the sink and tub/shower drains. While you dump, water can be sucked out of those P traps... so run some water in them.
5 weeks ago I switched to a product I found at Ace hardware... DRAINBO (Marine and RV treatment and cleaner).I'm really happy with it. Up to then I had tried RvDigestIt for about a year. And before that I was using TST Orange Tank Cleaner from Camping World.
We have a truck camper but we full time in a Class A. The Class A stays in one spot for very long periods of time. The last time we rolled these wheels was in October 2011. (The truck camper is used as a guest house and it's our "travel vehicle".)