I would first determine positively what tank odor you are experiencing. Give the gray tank a good cleaning. Fill it about a quarter full of fresh water. Add a good amount of bleach to kill any existing odors and to prevent any new odors. Leave the black tank as it is. If the smell continues, it will probably be the black tank. If it stops, it will be the gray tank. This will enable better troubleshooting.
I have experienced both odors in the cabin. The black tank odor was a bad seal on the water inlet to the toilet. I pulled the toilet and repaired the seal about a year ago and no tank odors still.
I at one point got a very bad gray tank odor. The odor was mostly due to temperature, the time in-between dumps and the lack of good odor control. I found that one of the drain line seals at the tank had dried out and because the pipe it serviced wasn't square to it, it began to leak odors and in some cases water. I replaced the seal with no additional odors in the cabin.
Since then I discontinued the use of advertised tank additives. I wasn't happy with anything I tried unless I used 55 gallons of it. I currently use a mixture of granular laundry detergent and pool shock in the black tank and granular pool shock in the gray tank. Both have produced much better results with good savings in the pocketbook. The gray tank health has increased tremendously using the pool shock. The blue stuff would just mask the odor. Pool shock eliminates it when used properly, easily stored and cheap. The black water will always be foul and relies on good plumbing and venting. It takes too much additives to neutralize the tank. I find that the black water odor will be fairly well controlled initially but slowly gets to its normal foul odor and usually well before a dump.
If the odor turns out to be a gray tank odor, look closely in the plumbing areas in the area you suspect the smell is originating. Use your nose under sinks, inside cabinets where plumbing exists, remove any access panels to plumbing areas and have a whiff. The adavents are a possibility. Each plumbing drain will have a trap. Each trap will usually have an adavent to protect it. Those can usually be found near the trap as high above the trap as possible. The adavent is normally closed but will open when dumping the tank if the tank vent cannot keep up with the flow of water that is exiting during a dump. If a tank vent is exceeded without the adavent, the vacuum would suck the water out of the trap and until the water is replaced, the trap will allow odors into the cabin. In my unit, both the bath sink and the kitchen sink have the adavents in the cabinet below. They did not put one on the tub which I determined as another possible odor leakage from the gray tank. I since upgraded my vent so to avoid that problem and to be sure I always run the water in the tub after dumping to be sure water is in the trap. In order for the adavent to be bad, It would not be sealing properly under normal water usage. I have never heard of one going bad but it is possible.
Black water odors are much simpler to locate. The toilet is always a prime suspect. The bad seal on my water inlet took a little time to figure out. I began to notice that the odor was less if the toilet cover was down. The blade seal seemed fine. I took a whiff one morning at the water inlet and sure enough, it was coming from there. I did a complete service on the toilet and sealed up the water inlet.