My trailer only takes 5 minutes to warm up, maybe 10 if it is really cold. It takes longer than that to setup camp and get situated. I think running the heater on the road is just a waste of propane but to each their own. Turn it on first thing when you get there and it will be warm before you are situated.
I have a propane detector and a carbon monoxide detector. I have no problem running my heater over night. Biggest problem with running it overnight is that the blower does drain the battery a fair amount... if you are running on battery power.