When you are hooked up to sewer do you leave your kitchen gray tank valve open? Over time small food particles will build up in this tank just like the poop pyramid in the black tank. Then when you close the valve either when boondocking or just traveling down the road, the water that you run into the tank will dampen the partially dried out stuff and you can not believe the smell. Worse than a black tank smell to me. I recommend draining your gray tank and adding a complete gallon of bleach. Drive around a bit if you can, but do not drain this tank until full. The bleach will kill the smell immediately and in my case it cleaned the tank level sensors as well.
After my round with this issue myself I always keep my kitchen gray tank closed and only drain when it is full or very close to it. I also add a pint or at least a glug of bleach every time I drain it so it won't get that bad again.