If closing the sink drains stopped the smell, that brings the suspicions, that you might actually have too much vacuum in your system, while driving, what sucks the water from the kitchen p-trap and allows the smell to enter on periodical reverse flows.
Did you take a look on the p-trap under the sink?
I encountered a similar situation. The smell was not from the black water but grey water tank. What happens is that when you drive you often lose the water in the traps at the sinks and tub. Without this water in the traps the odors are free to enter the coach. Now, I put a stopper in the bathtub drain while travelling (the sinks in the bathroom and kitchen don't appear to be a problem in my case).
Mr. Ed (fulltiming since 1987)
2007 Hitchhiker II LS Model 29.5 LKTG
2007 Dodge Ram 3500/6.7 CTD/QC/4X4/SB/SRW/6-speed man/Big Horn edition
That smell could be coming from your grey and black tanks. If your roof vent is clear of debris I would thoroughly clean both tanks. After each trip I also pour a half a bottle of Mr. Clean divided into each drain. That really helps keep the odors down for the next trip. I seen where some use vinegar and/or baking soda. You will get a lot of great suggestions. Good luck!
PS - I always put the drain traps in place after each use to keep the smell from spreading.
There are a lot of products, some people make their own.
I had the same thing happen in our last RV..the smell was awful on our first trip. I put baking soda in the drain and poured water and vinegar into the sinks and shower drains till I could buy something that worked better.
I currently use TST from an RV store, $10.79, in both black and grey water tanks. It works pretty good and keeps the sensors lubricated too.
If you are smelling sewer gas..that is a different problem.
(1) If you travel with windows or vents open in a way that creates a slight vacuum in your RV, you might get some gray tank air sucked back through your drains, even if the traps are filled. It just bubbles through the liquid.
(2) If the vacuum breaker for a sink (found in the cabinet under the sink) is defective, gray tank air might come up through it.
(3) If the tank is cracked, some odor might get out and enter your RV.
Other than that, there is no reason why any odor (and certainly not sewer gas, which is deadly) should get into your RV.
I expect my holding tanks to be dirty, unsanitary, and smelly.
There is no reason that I ever need to know about those "defects."
Aslong as there are no chunks to dry out, harden, and break off to clog the exits, they are doing exactly what they are intended to do: HOLD SEWAGE temporarily.