Mix in a couple of cups with a few gallons of warm water then fill the black tank 3/4 full then go for a nice curvy ride. Rinse repeat until the black water runs clear.
You could use a few cups fabric softener then fill to 3/4 full and let it sit for a few days then take it for a ride as a alternative.
2015 Jayflight SLX 287BHSW
2002 F-250 Extended Cab 4x4 6.8
Teconsha Prodigy P3 Brake Controller
Reese SC Weight Distrubition with sway control (12,000lb)