With the coach out of level the end effect is the slides will feel heavier extending or retracting. Your stressing the system a bit may be not excessively and possibly within design specifications but still a stress. Worse case scenario the weak link in the system fails such as a sear pin with an electric motor drive system.
As long as the coach isn't wildly out of level you'll probably be OK. My coach has two shallow living area slides and was originally delivered without jacks to level. Six seasons and two owners later I added jacks. Lots of extensions and retracts without a problem.