A couple things to check:
1. Make sure your roof vent exhaust is clear and not clogged with birds nest, etc. Should draw out the ordor while driving and spread the smell to those behind you.....ha ha
2. Make sure you don't have a leak at the drain connections (where you hook up hose to drain tanks.
3. Make sure you don't have a problem at your toliet (connections, etc.)
4. Of course, you need to make sure you drain and treat the tank.
5. Oops forgot one....make sure your grey tank valve is closed.