I live in southern Indiana and it has been pretty cold here lately around in the teens. So I have my truck plugged in when I park it and leave it overnight in the garage and today I started it and it warmed up fine and I went to drive to school and the temp just dropped when I got on the road, when I first got in it the heater was blowing hot air but once I got on the highway driving it was like having the ac on. Help?

 

The plug heater does not provide operational temperatures of the coolant. 40° above outside temperatures are realistic.
when you heat the road the wind chill comes into play. heater air is hit by extreme cold outside air and is cooling off real quick.
Check the normal driving conditions. Does the temperature come back when your engine is warmed up? If not you might have an open thermostat.
Just also have in mind that diesels take longer to warm up with no load and durin cold conditions.
Check and report back!


Just switch off the A/C J/K

 

Yeah, I found out that it was the thermostat.

 

Looking at the other 2 threads you started with this one, I'd say they are all related. It sounds like you have a low coolant situation. In your other thread you are 'adding coolant'. If the cooling system is not full and purged of all air, the t-stats and heater will not work properly. So first, you need to fill the cooling system while the engine is running and get it up to operating temps, check level and add coolant accordingly until full. Then, you need to figure out why the system was low and fix the leak(s). You could have a water pump leak, radiator, hose or even blown head gaskets causing coolant to excape the system.

It sounds like this truck is new to you, do you have any service history on it?