When you drain the black, is it showing a steady, brisk flow? It should. Since most RV black systems use the same 3 inch drain pipe, you can approximate the tank size from the drain time. When my black is full it’s holding about 40-42 gallons and it takes 32-35 seconds to drain. So as a general rule of thumb, every 10 seconds indicates about 12 gallons or so of tank capacity.
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John and Diane (RIP Lincoln, 21 FEB 22) RVM103 NHSO
Fulltimers since June, 2012
2002 Dutch Star 40, Freightliner, Cat 3126, 2004 Element
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