Dump the black tank first, then the gray tank, this helps rinse the hose.
Dumping the black tank first is considered best practice because the cleaner gray water can then be used to flush the sewer hose and remove any residue or solids left behind from the black tank. This helps prevent clogs and keeps the hose cleaner. Starting with the black tank ensures that solid waste is removed first, and the subsequent flow of gray water acts as a rinse, promoting a smoother and more efficient dumping process. This sequence also helps avoid contamination, as closing the black tank valve before opening the gray tank lever prevents any backflow. Additionally, many RV experts recommend this order to maintain hygiene and prevent unpleasant odors.