No, I don't add anything to minimize odors coming from the sink plumbing and grey tank. In both my SMB and my current van they are plumbed just like a house would be, with a P or S trap. The trap has water in it all the time and that seals off any odors from the rest of the plumbing or the grey tank. So from that perspective, trying to keep the grey tank from smelling (to keep the van fresh) would be like trying to deodorize the city sewer. There's no need because the trap seals off the tank from the living quarters. The tank is "supposed" to smell, and the water in the trap keeps it from getting into the living space.
RV toilets don't seem to have the usual traps (that was a new one on me, since houses and boats do). Instead they rely on the water in the toilet bowl and a sealing "flap" at the bottom of the bowl. So the bowl with water in it becomes the trap, so to speak. (Of course as you are flushing the "trap" is momentarily open --- good time not to have a Fantastic vent on "out" position...)
Yep, mods and upgrades are fun
I've noticed that some people don't like dealing with tank dumping. To me it seems easy. Pull up, don disposable gloves, let gravity do the work, rinse, peel off gloves, drive on. My only complaint on my SMB was that they put the greywater tank dump on the "wrong" side (passenger), as opposed to like 99.9% of other RV's that have it on the driver's side. So I'd always have to either go into dump stations "backwards" or else carry (and store) a long hose and drape it on the ground at the dump station to get it under the van (plus run around to the other side to work the valve, then back to the hose, etc.). That was a pain. But otherwise, no trouble.