We have been fulltiming since 2002, and I have used quite a few stinky-tank solutions.
On our last motor home, we were getting gray tank fumes in the coach while traveling. I solved that with an RV 360
vent cap. There are other similar products, and they probably all work.
For routine black tank odor control, RID-X works very well, but it takes a few days before it starts working as a deodorizer if you wait too long between treatments (7-10 days, in my experience). Until recently, I used it continuously for 6-7 years. It is very inexpensive compared to products aimed at RVers. The liquid version works MUCH better than the powder version for deodorizing. I have no idea whether it actually aids in breaking down waste in an RV holding tank. Waste appears to breaks down pretty quickly no matter what product I have used.
To reduce or eliminate black tank fumes, I found that Pure Power
works better than just about anything, works immediately, and is very inexpensive. I started using it exclusively about six months ago, when we met a lady in a campground selling it. It only takes about two ounces to immediately deodorize the black tank, and it's effectiveness lasts 7-10 days, like RID-X.
Another product that works as well as Pure Power is Odorlos
, but it is several times as expensive. On the plus side, it's pretty easy to find in RV supply stores.
Hope that helps.