Forget about the smell of the tank! The problem is that the smell is getting into the RV and it should not. Most likely as mentioned are the anti siohon valves/vacuum breakers. If the coach happens to be plumbed for a washer even if you don't have a washer there will be a trap that may be dry. Some toilets have an overflow that is at the top of the bowl, that dries out. Try filling the toilet to the top wait a few minutes then flush, that should fill that trap if it is dry.
Do you happen to have a roof vent near where the tank vent comes through the roof? You could be getting fumes in that way.
I recently discovered that my sewer compartment has a direct air passage into the motorhome. During a new A/C outlet install I removed all the drawers under the closet and over the furnace. To my surprise I could see the top of the holding tanks. This is also adjacent to the space under the refrige and all the circuit panels. If you have "Heated Holding Tanks" you may have the same issue.
I believe fumes that linger there can work their way into the coach after dumping. I now keep the sewer compartment squeaky clean and free of connectors/lines. (They all fit in the under chassis carrier now) I have also stopped trying to get as close as possible to the hole and use more sewer line than most.
Seems to have helped.
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