RV Fridge needs to be turned on 2 days prior to your trip, and everything you put in it needs to be cold already. Put all the items in your home fridge to cool, then move them to RV fridge. I won't get into the going down the road with the gas on debate. I have done it for years, others will say it is a mortal sin.
Rv fridges are slow to get going and slow to recover once the door has been opened for a bit. It helps to minimize the number of times you open the door. Plan ahead, get what you need, and get out of the fridge. For example, cooking burgers on the grill. Get the hamburger, cheese, onions, tomatoes, lettuce,and a beer out all at the same time. Cut those up and put your burgers on the grill. Then get out all of your condiments at the same time,and another beer, while putting away extra onion, tomatoes, and lettuce. Total 2 trips in the fridge vice 10.