"My Safari Simba, Workhorse gaser owner manual states: The motorhome must be supported by spring suspension"
Actually, I was very much surprised by that, too. The only thought I have there is that they want the frame of the chassis to be in its normally "deformed" shape due to sagging of the structure. That way, the adjustments to the fitup of the slide mechanism are consistant with the "as-built" configuration. If you support it on the jacks, those fits may be different.
Just a thought...............