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I am pulling this response out of thin air so be aware it might be a worthless response.

First, check voltages at the terminals on both Vanners.

Then disconnect the cables from the Vanner with the light. Without running the coach turn on the coach headlights for a while and measure the voltages at the terminals on the Vanner that is still connected.

What you should find is the 12 volt reading is exactly 1/2 of the 24 volt reading. After depleting some battery power by turning on the coach lights you should expect to see lower voltages, but the 12 volt reading should still be exactly 1/2 of the 24 volt value.

The tolerance for the voltage readings is 1/2 volt.

If the relationship between the 12 and 24 volt readings is not 1/2 the Vanner without the light might not be working and the one with the light could be carrying the entire load.