I think it is a waste of money.
My coach has four holding tanks and two vents with no problems.
I've had about four trailers in the past and they worked quite well as designed.
Keep the drain valves closed until you drain and then close again. If you leave open the smell will go up the vent and you will have sewer odor no matter what you put on the holding tank vent.
When in motion we keep our three roof vents closed, otherwise the vacuum formed will pull sewer odor into the coach.
2004 Monaco La Palma 36DBD
Workhorse W22 8.1 Gas Allison 1000, 7.1 mpg
2000 LEXUS RX300 FWD 22MPG 4020 LBS
US Gear Brakes