Our Freightliner chassis is already wired with a 12 volt circuit that is controlled by the ignition system. I just included wiring to the toad with a 20 amp. circuit breaker in series with the hot lead and it works great.
The wiring is standard issue for a 7 pin round connector. Would have to look it up to see which pin is the 12 volt pin but someone mentioned it above.
2012 Tiffin Phaeton 36QSH Diesel Pusher, TireTraker TPMS, Safe-T-Plus, BrakeMaster, '08 Honda CRV, Protect-a-Tow