I would change the air admittance valve under your sink for $20 first before tearing into the sewer system. Grey water smells as bad or worse than the black and you really can't tell that the valve is good by looking at it.
Also - check your batteries. Have read they will give off a very offensive odor if they are low on water and most confused it with sewer smell.
We had an issue of odor when traveling. DH found that the vent pipe had slipped down into the tank and wasn't really venting. He got on the roof and pulled it back up where it belonged - fastened in place and no more odor .
