2013 no heat from bunk heater

You either have a water valve on the engine turned off or an water control valve under the bunk not working, if it would happen to be cable operated(I kinda doubt it though being a 2013) you can turn it by hand. I would start by completely warming up the truck, and turning on the heat in the bunk, the go out and get under the bunk from the passenger side and feeling both the water lines under the bunk If 1 is hot and 1 is cold then the stuck valve is in the bunk HVAC somewhere. If they are both cold there is a valve closed/stuck upstream some where usually on the motor. FWIW my old T600 had 6 valves on the engine to control hot water flow. I'm sure these newer trucks have even more valves. Make sure they are all open, could be as simple as a mechanic closing a couple valves to replace a hose or filter or something, and forgetting to turn them back on.