If you have a Dometic fridge, than their manual addresses the question. Here is a copy of the User Guide for a Dometic DM2652
Two points are important here:
1)... the vehicle needs to be leveled only so it is comfortable to live in (no noticeable sloping of floor or walls).
So, basically, this gives you an idea of what they consider to be "level". If sloped badly at the restaurant, than the second item is that you have some time before it becomes an issue.
2) ...Any time the vehicle is parked for several hours with the refrigerator operating, the vehicle should be leveled to prevent this loss of cooling.
In my own experience, I have never had to shut if off and it has never lost its ability to cool because it was not level during a stop.