Do not know why Coleman would say that unless it gives them a "Out" so that if the unit is so out of level "Front to Rear" that it causes problems with the unit.
Both our A/C units are not mounted level, the rear living area unit is lower in the back and the front bedroom unit is lower in the front. See Pic
Most A/C units Travel Trailers are mounted level due to the fact Travel Tailers have a "Flat Roof"
Many A/C units on 5th Wheels are not mounted level "Front to Rear" due to many 5th Wheels have sloping roof lines.
Our 05 Eagle 5th Wheel A/C unit was not mounted level "Front to Rear" either.
Looking around the campground we are in this week, I see many 5ers with A/C units not mounted level "Front to Rear"