If your truck is modern enough to have a trailer wiring package, the hot wire is already installed. Just run a jumper from the breakaway batt on the trailer to the winch batt.
Otherwise, starting at the truck batt, using #10 or even better #8 wire, run a 30 amp breaker, relay, & line to plug. Wire the relay so it is controlled by the accessory side of the key. This way the trailer batt will always be charging while driving, won't draw down the truck batt while parked, & won't overload the ignition switch. It only takes about 1/4 amp to trigger the relay while the contacts carry the load.