for the most part they are a wallet drain.
its true that positive ventilation in a waste tank helps to quickly break down the waste, but "quickly" is relative.. unless its an unusually large tank where it will set for quite awhile, it will need to be emptied before the positive ventilation has a chance to make a difference.... and if the system smells so bad inside the coach that it needs positive ventilation to suck the odors out, there is a problem elswhere..... and unless there is a double vent in the system where fresh air can go in, how is the device going to suck anything out the tube its mounted on?