Venting Under The Sink???
Sounds like there is something wrong there. Vent stack should run all the way up and out on the roof. If it's exiting anywhere else, there is leak in the vent stack or the vent stack is clogged and venting where it shouldn't.
Most people who get into trailers are surprised to find that their "grey" water smells much worse then the "black". This is because we tend to "treat" the "black" water while leaving the "grey" tank to fend for itself. This allows for a good bit of pent up stink. It is a good idea to rinse the "grey" tank often and treat it with chemicals (different one then the ones used for "black" tank). K-Z recommends rinsing the "grey" tank with a mix of clean water and Dawn dish detergent. I like to do this at the end of a long trip and at the end of a season. I fill the "grey" tank about 1/3 full and add a couple of real good squirts of Dawn. I then just pull the rig room and put it in the drive. When I have the time, I drain the tank, add my chems and go on.
Hope this helps and,
Turn Key, DW and Pepsi, The Camping Boston Terrier!
'03 Chevy 2500HD, X-Cab, Long Bed, 4X4
'04 K-Z Durango, 275RK ("Sunday Haus II")