I have found by experience that using black tank treatments is a waste of money, especially if you move your camper opposed to sitting long-term on a season site.
If your camper is on a seasonal site, then yes. I think the treatments help cut down on odor. But I have found that just flushing the tank keeps odors at bay just was well.
If your camper is in "transit" (not seasonal), then probably one of the best ways, in my opinion, to keep it clean is to dump everything before hitting the road. Then add about 5 gallons of water and about a half a cup of Dawn Dishwashing liquid. The bouncing on the road and splashing of the water in the tank will clean the tank and keep it from smelling. Do this every time you move the camper from home to campsite, to campsite, to home, and you'll never have smelly tanks.
Other folks use different kinds of dishwashing liquid, laundry soaps, floor cleaners (like Spik n Span or Mr. Clean products) and even some bleach. Be careful if using bleach, you want to flush your tank after bouncing down the road real good. Bleach is not the best for your dump slide valves and over time can cause deterioration. I use bleach very seldom in the tanks, as Dawn does a great job deodorizing and cleaning.
I will use the black tank treatments over the winter months when the camper sits idle, with RV antifreeze in the black tank. This is about the only time I use it.
Holding tanks are not septic tanks. They only hold the contents. If you experience a poop pyramid, filling the tank with water and Rid-x and letting it sit for a month will help dissolve the pyramid and break it down. But as a treatment between dumps, every few days ... nah... you're tossing your money ... literally ... down the toilet.