The black tank is not connected to the gray tank. The gray tank is the shower, sink water. The black is toilet only, so the P-trap from the shower or sink will not contain any black tank odors, though the gray has a bad smell all it's own.
Gross to drive around carrying your poo anyway, but the black tank is your poop. The vent is directly open to the black tank.
It is there #1 to vent gases bc the poop decomposes and gives off gas which expand and would otherwise inflate the black tank, or bubble up through your toilet, or try to, and would blow a geyser when you flush (open) the toilet...
#2 The vent also allows the black tank to empty fast as the vent keeps the outflow of material from creating a vacuum in the tank. You have a vent on your house plumbing also.
Black tanks stink. The fumes coming off the roof vent stink, and they can be smelled.
Keep the blue stuff in there, use the toilet sparingly, and empty and clean as often as possible.
If it is coming out of the roof vent and into your cabin air vent then, yuck, and moving the black tank vent outlet further away from the air vent will help.
Smell is not coming from the sink or shower.
The toilet gasket could be failing and letting smell out, but most likely is just the roof vent odors you smell.
I have walked by plenty of RVs and smelled their roof vent stack blowing slight odors of yesterdays chili a few times....
