We have a 2013 Itasca Meridian 42. I have found that I don't get heat once the temperature really gets cold. I called Freightliner on this, and they indicated that what I was describing was NORMAL (really???), as the engine wasn't really working very hard (fully loaded going down the road). Had the mechanics check it out anyway, and they confirmed it was fine.
Friday morning, I had to take it to the shop, and outside air temp was 51 degrees. The unit has a 180 degree thermostat on the engine, but on the highway, I found the engine temp to vary between 176 and 184. In general, this unit doesn't really start throwing heat (like in your car) till it gets up to around 195. And it just doesn't seem to get there in the winter.
You didn't mention your specifics (model, etc.), but that's what we've seen in our unit.
Cliff