1. Never open both tanks at once. You don't want black tank waste back-flowing into the gray tank. Empty the black tank, then put a flush wand down into the toilet (after turning off water system so toilet doesn't continually flush). Move the wand around and keep flushing tank until clean. When clean, let it empty, close the valve. Then open the gray valve and let that water flush any residual waste out of the drain hose. When the gray tank is empty, close that valve. Disconnect the drain hose and flush it clean with a hose into the sewer.
For sanitizing fresh water, mix 1/4 cup of bleach for every 15 gallons of fresh water capacity into a gallon of water. Dump into holding tank (never dump full strength bleach). Fill the tank to overflow. Pump this through your plumbing, both hot and cold. Let it dit for several hours, then flush completely with clean water. There are some great You Tube instructional videos on both these topics.
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