The odor may be from a different source than you think. It's very common for toilets flushed with raw water
to pick up biological stuff, critters and plants, that get trapped in the rim of the bowl. They rot up there and the smell can be horrible. The cure is to remove the bowl from the boat and flush the rim with a hose. try to get all that stuff out. It's not easy because of the small holes in the bowl rim. The prevention is a good strainer in the intake line and some times stuff gets through that.
It would be very difficult for odor to get back into the toilet from the holding tank. There is already a flap valve in the discharge of the toilet. It's possible the flap valve is bad. If so replace it. I wouldn't add another check valve, you would just be asking for a blockage.
The discharge hose may be saturated and need replacing. Rub the hose with a rag. Sniff the rag. If you smell sewage, replace the hose with Trident or Raritan
When you flush, be sure to flush long enough for the sewage to be pushed all the way to the tank. Most people just flush long enough to clear the bowl. You don't want poop and piss sitting in the hose. The hose eventually gets blocked. If you're worried about holding tank capacity, at least give the line a good flush at the end of the day.
Read Peggie Hall's book for more information.She explains a trick for using the sink drain to supply fresh water to your toilet intake once in a while and that helps with odors as well. Here is a link to the book: Get Rid of Boat Odors by Peggie Hall
I see SailFast beat me to it! Great minds think alike.