In my opinion, the variety of after market vent caps offer more form than function and then it is one's opinion if they look better than a standard cap which is fairly unobtrusive to begin with. If you think about how the black tank is plumbed, you will note that it is basically a closed system most of the time with the dynamics of an open half empty milk jug. The vent is used primarily when the toilet or dump valve is open and with the latter, it is letting air into the tank. As air passes over the roof, a bit of mixing occurs at the very top of the vent pipe, but without a secondary source of air, very little of the smelly gas escapes.
This is also true of the gray tank. Many leave their gray valve open when the slinky is connected to the park's septic inlet. If the septic has another source of air and the slinky doesn't form a trap and the wind is blowing the right way, you'll vent the park's septic right out your gray vent and things won't smell so good around your trailer. Close the dump valve and all that venting (at your trailer) ceases.