My 2014 Bighorn V6 has the same issue. I traded a 2011 with the 4.7 that warmed up very quickly and would have to turn the heat down after only a few miles. My new one stays on high all the way to work (about 35 miles). I have seen a post on here about the new ones transmissions using a heat exchanger to help them bring the transmission up to temperature quicker for better fuel efficiency. I have seen a picture posted that showed the heat exchanger lines run on the same lines as the heater and if this is the case I could see how the heater would take longer to warm up.
Long post short, my heater also takes to long to warm up.