RV Repair: Holding Tank odor inside., creative plumbing, bathtub drain

Question
I have a 2002 Jayco 285BH fifth wheel. I get a odor inside the unit sometimes from the gray water tank. The odor comes from a vent / check valve that is located inside the electrical compartment by the bathroom. The vent is "T" into the bathtub drain. It is a screw in fitting with a rubber diaphram checkvalve. I assume this is to help the water drain better out of the tub. The odor is coming out of this pipe and checkvalve. I do not see anything wrong with the valve. I taped up the vent and I do not get the smell anymore. What is the purpose of this valve and is it necessary?

Answer
Mike,
    The only solution I know of is to run the pipe up through the ceiling and put a roof sewer vent cover on it.  Depending on where the line is located this may require some creative plumbing but that should eliminate the problem.  

Darren