Addition information
I finished my installation last night. I used the DrawTite 18381 connector to plug into the factory wiring. I cut off the flat four connector and used the ground, tail, and turn wires to lead to my inbed receptacle. I had to solder a pig tail lead off the brake wire of the drawtite adapter because a flat four does not have a brake lead. I ran a #8 wire from a terminal at the driver's side battery for the battery lead (protected by a large circuit breaker) rather than use the factory battery lead which is too light for the job. It also supplies an receptacle for my boat trailer winch. I put a heavy ground from the bed receptacle to a common ground on a bolt on the underside of the bed near the rear bumper. The ground leads from all the receptacles terminate on the bolt. The advantage of the DrawTite connector is that you can do all this wiring without splicing/cutting into any of the factory wiring. The only thing I don't have is an isolator to separate the trailer batteries from the truck batteries, so the trailer batteries will probably never get a full charge from the truck.
Jim