The tech at my dealer recommended either being plugged into shore power or having the generator running when extending/retracting the slide.
I think the idea here is if the house battery isn't fully charged or is wearing out, the voltage can sag when running the slide motors and potentially cause them to stall/amp out before reaching the end of travel. On shore or generator power the battery is getting a big assist from the 12V converter.
I can hear a definite change in motor pitch when I don't plug in before moving the slide and it sounds like the motors are working harder. My slide is quite deep and the full length of the coach so I imagine its quite a load on the motors as it is.
It also helps that my shore power cord is underneath the slide so it's much easier to get it out and plug it in before extending the slide and unplug it after the slide is retracted.
So, it'll work fine not plugged in but where convenient or when your house battery is low it's a good idea to plug in first.
It will work fine just using the battery but as mentioned it depends on battery charge and condition. I prefer to extend/retract either hooked to shore power or running the genset. I find that the slide motors run more evenly and the slide seems to go in and out a lot smoother. I did however upgrade to two house batteries in case of SP or genset problems as an insurance policy. I ďtestĒ the slide operation on batteries every 5th or 6th extend. I almost always retract on SP or genset. I like a snug slide while moving.