Yes you can run the furnace in a 5er while driving down the road or while not plugged in. If you couldn’t, boondocking in cooler/windy weather would not be possible.
We travel from PA to FL the last two weeks of Dec almost every yr.
We have a programmable thermostat.
We leave on Fri just after rush hour.
I have the thermostat programed to kick on about midnight.
I’m usually ready to pull over about 2am.
If you don’t have a programmable one you could just turn it on during your last pitstop before you know you’ll be stopping for the night.
Dave, Judy & Daisy May
2010 Sierra 356RL
2011 Silverado 2500 4WD, LT, Ext Cab, LB
Duramax 6.6L V8 Turbo Diesel
Allison 1000 6-Speed Automatic Transmission