Just returned from a long trip to the western US (again). I discovered that I am not charging my RV battery through the trailer plug (factory towing option) I get no 12V through the + terminal on the trailer receptacle, ignition on or off. Do I have a problem or do I have to do something else to get this to work. I know that an isolation solenoid is necessary to keep the RV from running the truck battery down, but I've been unable to find out if the factory towing package includes this.