According to Dometic;
Any time the vehicle is parked for several hours with the
refrigerator operating, the vehicle should be leveled to
prevent this loss of cooling. The vehicle needs to be
leveled only so it is comfortable to live in (no noticeable
sloping of floor or walls).
When the vehicle is moving, the leveling is not critical,
as the rolling and pitching motion of the vehicle will pass
to either side of level - keeping the liquid ammonia from
accumulating in the evaporator tubing.