Your valves should always be closed while on the road. While camping, I used to leave the gray open until the day before leaving and then close it to build up water for flushing the hose after dumping the black tank. But I realized that deposits were more likely to build up from light water flow, so now my policy is to dump full tanks so the momentum flushes deposits (shaving cream, soap suds) out leaving a cleaner tank.
I have a built-in black tank cleaner and turn that on a few minutes before dumping the black tank to add more liquid and dilute the tank. I leave it running while the tanks dumps until it runs mostly clear (clear plastic adapter at the outlet). Then I close the valve, let the cleaner run another 5-10 minutes and open the black valve again. Amazing how much more debris comes out. If not in a hurry, I do that one more time and let it run for a few minutes with the valve closed just to have some starting water in the tank.
Once done with the black tank, I empty the galley tank next as it may have food debris in it and finally the shower tank to flush any black or galley tank debris from the sewer hose.
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2015 Ford F-350 XLT Ruby Red 6.7L Powerstroke, 4x4, Short Bed, Andersen Ultimate Hitch
Cougar High Country 299RKS, Mor/Ryde pin box, 200w solar
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