To the best of my knowledge, NO black tanks are "interconnected"... It would be "impossible" to get the solid waste to flow between tanks or to "empty without the rush of water as the tank empties"... So, each tank will have a separate 3" dump valve. The tanks "may" empty through a common sewer connection on the side of the trailer, but each tank will be controlled separately...
Also, I don't know of any trailer with the black tank rinse system connected to both tanks so they are rinsed together.... To do so would overfill one tank while rinsing the other (or require both dump valves to be open during all rinsing operations)... If there are two separate sewer connections on the trailer (for two sewer hoses) it would make it "almost impossible to rinse tanks" at most dump stations without installing a complicated sewer hose system for a simple tank dump....
If your second black tank does not register on the control panel, and ALL the lights remain off when the button is pushed for that tank, I'd suspect the ground wire at the tank is loose, damaged or cut.
The way your system "should be plumbed" is with separate black tank valves, separate black tank flush connections and "possibly" a single sewer outlet for all black/gray tanks" Typically, the "main tank valves" are in the convenience center and the second bath valves may or may not be located in that same convenience center. Some are, some are installed closer to the tanks to prevent excessively long Bowden cable runs.
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John
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS
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