When you are driving...that's when you find out where the air leaks are, areas that aren't as insulated as we'd all like and the limits of dash heat versus propane furnaces.
Your dash heat should make a big difference but if you don't feel warm enough, I agree check it out for problems. But it's more than heat out put...
The area around entry doors are notorious for air leaks, front caps usually don't have insulation behind cabinetry and windows will feel colder and maybe you'll even feel air leaks from the window track weep holes. Also check all around slide-outs, especially at the floor, around pipes, power outlets, etc.
We traveled one year in December and January in the west and northwest in temps as low as zero. So, we used our mirror heaters for the 1st time and ran both suburban furnaces the whole time along with the dash heat. At one point, you betchya I wore my insulated coveralls and winter boots indoors sitting by the entry door.
After your 1st cold drive make adjustments to create a comfortable environment...put on warmer clothes, and yes, turn on those propane furnaces. Not only will you be more comfortable but the pipes and plumbing in the utility bay will have a better chance of staying thawed!
2002 Tiffin Phaeton
2005 Malibu Maxx toad