Bayes is right, big difference between 180 and 200. I've run my diesel car around plenty with a snap-on scanner attached. I had a bad thermostat and the engine was running at 160. It'll throw what feels like good heat when it's 40 degrees outside, but extremely weak at 10*. At 180 it 'feels' warm but takes a while to warm up. Between 195 and 200 it's nice and toasty. Over 204 you're starting to get a little concerned. During a short commute in a diesel, you'll be lucky to get the engine very warm. Does your tk allow you to view detailed engine temperature?
You might want to have someone drive the truck when it's warmed up, and sit in the passenger's side and feel around for any cold air coming into the cabin. There's always a possibility that an electric HVAC motor has become damaged ...or the outside air door isn't shutting. On the motors, it's fairly common that the female part of the plastic motor shaft that goes 'around' the square male hvac door shafts ...becomes cracked due to stresses and cold temps. The motor can function just fine, but if the motor shaft cracks it'll 'spin' on the door shaft which causes it to 'hunt' because it's trying to find it's 'closed state' so it knows where it is. During the 'hunting' process it can let in cold air. You'd generally hear it clicking as it spins on the blend door shaft, but maybe not if a piece breaks off. The outside air blend door, right above the passenger's feet is a constant source of problems. As a driver you can't feel the cold air coming in, and 0 degree air will do a great job neutralizing heat from the HVAC system. But as an astute passenger you should be able to feel the cold air with your hands to see if there's a problem. It's probably not the issue, but worth a quick check.
