Shirley, (Colusa, CA)
For improving upon the design elements of something as nondescript as a sewer vent, you may want to check out a product on the market called the Xtreme Vent; http://www.coilnwrap.com/more/Xtreme-Vents.htm. I'm a big fan of using them on both black and gray holding tank vents.
And rather than masking odors from the holding tanks, I recommend using a biological additive, one that employs live bacteria to literally eliminate the odor-causing molecules themselves. Many suppliers have now produced such an enzyme-based additive. Avoid using any "chemical" in the holding tanks, especially those that contain formaldehyde. Detergents and some ammonia products may damage seals in the termination valves.
Contrary to what even I have expounded in the past, also avoid home brews. Realize suppliers spend lots of money on R&D, so why risk the integrity of the RV waste system for the sake of a few dollars. Once you switch over to the live "bugs," further use of other types of deodorants and/or detergents will likely kill the good bacteria and defeat the purpose.
As for evacuating the tanks, many RVers completely close the black water tank and evacuate when it approaches full capacity while leaving the gray water valve fully open so bath and kitchen water freely drains the entire time. The drawback to this method is that obnoxious odors can and will proliferate and emanate from the gray tank quite rapidly. I support the method of keeping both container valves fully closed and evacuating each when almost full and using the live bacteria additive in each tank.