How close must you be to true level to not harm refrigerator

Here is something for you to think about. How many of us leave the fridge running while traveling down the road? I know it can be dangerous, especially when stopping to refuel. But many of us do it, especially when making a long drive from one campground to another & it's in the summer when temps are hot. Is your fridge level then? Can't be for more than a second or two.