You dont say what coach/model you have [suggest you add it to your sig blk] but there are acouple places for oders to "occur"--doubt if its the outside vent. Check all p-traps; second, check the drain pumbing for the washer hook-up in the coat closet; third, check the connection where the grey water from the kitchen sink goes thru the wall opposite the shower stall [ie, the wall where the control handle and spray head are mounted]. The sink drain connection is known to break or disconnect between the wall and the kitchen/room slide....
PS--think grey water doesnt smell bad--think again....
2003 Alpine 40' MDTS
New Braunfels, Texas