After emptying and flushing our black tank at the site, we always add a bag (or two) or ice into the tank thru the toilet for the drive home (or to next site). The sloshing of the ice while driving down the road will loosen things up and the ice melts to keep a small amount of water in the tank. By the time we're home (or the next site) - we add the deodorizer chemicals for storage.
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