[jimcol]

My kitchen sink is plumbed exactly like yours and has the vent in the wall that jsb5717 refers to. When I had odor in the kitchen sinks it was the defective air admittance valve. I don't think the slope of your crossover pipe is the issue. Standard pluming only calls for 1" of slope to 4' of pipe. If you think about it gray water is what stands in the standard P-trap and it normally doesn't smell.
I've never known gray lavatory water to smell unless there was build up of hair in the down pipe. (Most always on the women's side of a double sink vanity.) You can remove the stopper in that side and clean the tail piece down to the elbow.