When you plug your RV into your Truck, the Truck provides another 12-VDC source for your camper. This could signify that your COACH battery is not fully charged. In fact, it may be low on water or it can be bad.
Having 13.7 VDC across the battery terminals is normally an indication that the battery is charged and the RVs Converter has done its job, and that your battery is adequately charged for your slide, awning, and jack to work.
You may be getting a false reading across your battery if you still have your Solar system connected to it, so disconnect the solar system and then try to diagnose what the problem may be between your battery and your Converter.