Must be a decent block heater in this Ram. This engine takes a life to warm up the coolant when running. You might be saving 3-4 mpg during the first 15-20 minutes on average potentially with daily block heater use. Are you going to find the minimum on time to reach 40 C with -18 C ambient? That would be great data!
The defrost and cabin will get warm within minutes after a cold start due to the 900 W PTC heater in the HVAC system. Think of it as the heater in a hair dryer. The high idle most likely compensates for alternator load (steering wheel, seats and PTC heaters are pulling several amps.)