As little as 5 minutes off level can send boiler temperatures skyrocketing.
At ARP seminars I have attended at FMCA Conventions, he has an absorption refrigerator cooling unit with a digital thermometer on it that displays on the screen.
He tilts the unit and you can watch as the temperature goes up. No sales hype-- you can see the affect of running out of level.
As many has said, the damage is cumulative. One "out of level" is not going to kill the unit.
But, we have been on many L O N G grades out west where you would not want the refrigerator to be running. The ARP controller automatically turns off and back on the refrigerator based on boiler temperature.
And, whether this was a "lie" or just an uninformed sales person, I have no idea.
But, your refrigerator owners manual is a much better source for how many degrees out of level will affect the cooling unit than any sales person.