Black Tank Roof Vent and Bathroom Roof Vent Too Close?

The sink may not have a vent. Sometimes the sink will have a one-way air valve so when water drains down from the sink it can pull air into the pipe and not suck water out of the p-trap, but possible it has nothing. Otherwsie it should have a vent pipe connected to the roof vent pipe.
The black and gray tank's vent pipe might connect together before exiting the roof, so if you just have one vent stack on the roof then they are connected, not a problem. They would be connected as high as possible, and odors should not rise from the black up to the connection and then back down the gray into the cabin. The gray pipes are all blocked of by water in the p-trap, so after a drive make sure there is still enough water in the traps (sink drain) to block off airflow.
It is possible the water gets sloshed out of the trap enough that air can get by, and just 2 seconds of running water in the sink will fill it back.
Also, if the wind blows really hard and the trap is empty then possible odors swirl around in the vent stack and move back down...
In a double sink only one drain might have a trap, not both, only one is all it needs to block air from the gray tank, not so much the pipe. The first drain flows into the second drain pipe then into one p-trap. The p-trap could be down stream a few more inches than normal if the camper cabinet is tight on space...
You should not need a drain plug, the p-trap will do the job.
Some people have worse smelling poop than others also, some have very unhealthy guts, and not much the camper system can do about that. Try to not use the bathroom in camper if you can help it.

I now see that the grey drains do not go into the black tank at all, so the black is all on its own. Not sure how the sinks and shower drains are vented. Double kitchen sink so that might be it for there if you don't plug both. Bathroom shower could vent via bathroom sink not plugged. No air admittance valves anywhere in the pipes past the Ptraps. (1991 RV)

Good idea for a shower drain plug. You have to stand in the shower pan while at the bathroom sink. This is not a 5er! :)

Got the drains all cleaned out so have to do a smell test later. The black tank is all cleaned out too so be a while before any tests.

I have to rig a second tank under the TC overhang behind the truck bumper to be the grey tank. I did that before with our previous TC. Fairly simple job where it can be hanged from above and supported below with a towing piece in the receiver with a plank over to the side attached using the hole where the ball normally goes. Just have a short hose that goes from the sani-cap down to the grey tank. A tap to dump it if it doesn't have one.

A plastic bigger gasoline can works for that if on it side. Its pouring hole on top to take the grey filling hose, and leave its vent screw- on cap at the bottom to be unscrewed as the drain. Good enough for sinks, but not big enough for shower of course.

I vote for a fan. You want to cool the TC anyway during the summer. You can route the vent pipe further away but wind direction is never consistent. Can make one yourself relatively cheap with a computer fan and a piece of insulating foam.

To test your air valve wrap a plastic bag over it. I did that to my trailer 2 or 3 years ago to test my valve as the cause of odor. Just remembered its still on there!!!

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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....

I remember you saying you have a shower drain. It should have a P trap under the floor. If it dries up, fumes from the tank can come up through it. To avoid it from allow fumes you may seal it with one of the flat drain stops.

Thanks for the ideas. I am working on confirming the source of the whiff, which will take a few days. I have closed the roof vent and cleaning the drains more to see if there is any whiff after that, then if not, I will open the roof vent and see if any then to confirm my original suspicion.

The whiff is definitely black not grey when you can smell anything at all. The tank vent on the roof has the usual rain cap. The bottom of the vent at the top of the tank is visible and sealed, so it is not there. None of the grey drains has an air admittance valve, but that may not matter if there is no smell with the roof vent closed. I will know about that later.

It will all get solved after a while. There is no rush, and thanks to all for things to check.

Are you sure it's not coming from the air admittance valve under the bathroom sink? They do wear out and will allow tank fumes into the rv. The grey smells as bad as any black. They are usually 2 or 3 of them in your plumbing. About $25 at any big box stores. Try that first.

Try an RV Venturi Vent cap/top on your black tank vent and/or gray tank vent. That fixed that problem on our '88 Mallard's black tank.

I have MaxxAir II vent covers. If they were on backwards, I think the amount of air trying to be rammed into them at 65 MPH would rip them off the roof. My bathroom vent has a fan so I don't have your issue. If at all possible, I think the addition of a fan would be the best solution.