Has anyone tryed one of these holding tank vent filters? I've seen one made by Vetus for around $50 and another brand that was about $80. Both claim to last a year? For the price i should hope so! Just curious if they work, after installing all new heads and holding tanks not smelling the odor when flushing would be nice! By the way I've tryed several types of tank chemicals but they all still seem to still fume out the nasties!