Our secondary braking system has 6 wires from the motor home to the tow vehicle. One of them is an extra hot wire to the secondary braking system in the tow vehicle, therefore leaving the tow vehicle battery alone.
If your tow vehicle's electrical cable has only 4 wires as most do, you can utilize the RV ignition switched hot wire from the RV connector. Utilizing power from the RV would require you to either replace your 4 wire cable with a 6 wire, or add a second cable. Either way, you'll be running the hot wire from the RV to the secondary braking system. All the wiring modifications are on the cable and tow vehicle, no modifications to the RV wiring because it's right there in back in that big 7 wire round connector.
I just been through all this last July installing a US Gear Unified Tow Brake System. The Unified installation wiring diagram CLICK HERE
should help clarify my description. The RV hot wire is the red 14 guage wire labelled umbilical cord. It illustrates a separate 2 wire cable, but I changed my 4 wire to a 6 wire to eliminate the extra cable.