Could be a valve behind the washer, ours has one.
Drain hose: It needs to shortened enough so it does not protrude past the pee trap on the washer drain. It will only allow odors if it goes past that trap. Ours did, shortening solved the problem. Shorten it so about six/eight inches goes into the pipe. Secure it with a tie wrap if you wish.
The other reason for odors can be that the pee trap is dry. This happens because not everyone uses their washer as often. When storing or if you do not expect to use the RV washer for an extended period you can put anti freeze in the pee trap and it will not evaporate as fast as water. Running the washer, of course, leaves the trap filled.
Another common reason for odors is that when you have a fan on like a fantastic and another roof vent open odors can be drawn into the coach from your roof vent pipe. In one vent out the other, or drawn from that pee trap because the pressure inside the coach changes enough to draw the odor.
I do all my own stunts
03 Dolphin LX 6355, Workhorse W22, 8.1 vortec, 04 CR-V, Blue Ox, Brake Pro----Norm, Barb and
Doc(He's a PhD)