Sounds like more than a closed winterizing valve on the hot water heater. But that would a place to start. Otherwise I would be back to the dealer and let them deal with it. Another good reason not to rush PDI.
2017 Chevy 2500HD LTZ DuraMax Diesel Silver Ice Metallic
2017 Outdoors RV Timber Ridge 25RDS Mountain Series 4X Off Road Suspension Pkg