Rid-X for septic tanks doesn't do much for RV black tanks.
Rid-X RV might do something for RV black tanks.
Those who say RV black tanks are just holding tanks, stuff is not in them long enough for any biological action to occur, and water is all that is needed, ignore what can happen in some black tanks.
As soon as your bacteria laden sh!t is deposited microbial action continues. In the absence of sufficient oxygen, anaerobic bacteria will immediately flourish causing foul odors. Promotion of aerobic activity will reduce odors. Black tanks used and then emptied less often than a few days can accumulate buildup that when emptying occurs then hardens.
Hence, no one can definitively say that water alone will eliminate or reduce black tank odor and buildup problems in all black tanks. Treatments that promote aerobic activity and breakdown wastes do have value for many RVers.