On our 06, the slides operate off the single chassis battery, not the 2 coach batteries. My guess is that it is easier to do the safety interlocks of the one 12v electrical system. I think they recommend having the engine running just to prevent running the chassis battery down and to make sure the slideout motors get adequate voltage power.
On ours, the slides work without the engine running, but a bit faster with the engine running. They will not work, if the parking brake is not set.
Ken