We fire it up a couple of days before we leave on a trip. Put stuff in the second day and leave the next morning. That way, all the opening and shutting the door is done and everything can get to a good nominal temp.
We always plug in when we do this because we also want our batteries to have a good 48 hours to be absolutely charged. Then, we run on 12vdc going down the road and then on propane when we get where we are going (most of the time, because we boondock). I check the refrig on propane every few months year round just to make sure it is operating correctly.