If you pack it fairly full and get everythjng nice and chilled then unplug just before leaving , everything will stay cold for 3-4 hours easy.......even in 90 degree temps.....
One of our camping friends throws a chunk of dry ice in the freezer portion and says it stays way way cold!!
Now to your original question, on my old lacrosse I had installed a 1000w inverter anyway, so I had thought about plugging it in while driving, but it was 30+ feet from my inverter....didn't feel like running cables since it stayed cold on trips anyway...
My new rig has a small outside fridge and a residential fridge and both run on the 1000w inverter. ...the small fridges don't pull many amps/watts except at startup...
2015 F350 Platinum Fx4 6.7 Diesel Dually. B&W turnover, B&W Companion, air bags and wireless controller.
2015 Heartland Road Warrior 420 Rt
2012 f250 Fx4 6.7 Diesel crew cab tinted n tuned
2012 lacrosse 318bhs touring- sold