I agree with MLP. I have an RVi brake, professionally installed, and my towed (F150) battery died when I towed with a one night stop (two days connected without any changes or disconnects). I installed a kit (under $100) to charge my towed battery from the 12 volt line from the motorhome, and have had no problems since then, even during a three day tow with no disconnects or F150 starts or disengagement. My cable is a 7 pin connector on the MH to a 6 pin connector on the F150; all wiring is through this cable.
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