The thinking that a motorhome will just slid away while on jacks without the wheels touching the ground escapes me. That is like saying if your house is built on a sloping property it could slide downhill.
Once the unit is on the jacks the pressure is down not lateral. Any gravitational force is gone since the coach has no wheels to roll on. How much of a slope would it take to make something like a motorhome slid downhill on its' jackpads? Without a practical test I can't say positively, but would think that something in the area of 15+ degrees of slope may be something to think about. Fortunately that is a degree of slope we would simply not be on in the first place because we would know we couldn't possibly achieve level before we tried.
Maybe my limited understanding of physics is wrong, but that remains to be proven by logic, not conjecture.