I chose the house batteries because it was more convenient. Actually I was trying to kill two birds with the same stone. There was no lighting at the rear of the coach so I used the same pair of wires to power a string of LED's from the house system. If it doesn't work out or if the voltage is too low at the towed battery I will move the charging pair to the back of the 7 pin. I really never considered that the house voltage would be inadequate before reading all of your posts.
My tow cable was only 4 conductors provided by the guy I bought my Blue Ox tow hitch from. The 6 pin connector has two unused. I had a choice of changing it to 6 conductor or running an extra pair of wires for charging, which I am doing, for now.
