I'm with bobbelcher. However, some coaches in some years have less than rigid front cap and chassis construction and lifting the front too far can result in problems such as a popped windshield. That is seldom a problem with better models or any more recent year, but its a case of "your mileage may vary".
Ditto for getting the rears off the ground. How is the coach gonna roll with the jacks firmly planted and held down to 10-20 tons of coach? Of course, a jack could fail, and once again that was more of a problem 20 years ago than now. Remember the HWH knee-action jacks? They could fold under sideways pressure. And some lower-priced rigs had barely adequate jack capacity for the weight.
My best answer is that if you aren't sure your chassis and jacks are up to it, better not do it. Safe is always better than sorry! Carry a few wood blocks and drive the wheels up on them if jacking lifts them off the ground. It only takes a few extra minutes.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC