What is the best way to de-ice slide-outs

I think you answered your own question, BDSUSA, with a good idea. Prevention is the best way, like pulling the slides in before hand, but it's not always possible. With sub-zero temperatures, ice will be tough to deal with.

One year out in SD, I had seen campers' departures delayed waiting for the sun to come out and thaw things out a bit. And some were equipped with slide toppers. The toppers sagged with the accumulated rain water and froze to a block of ice overnight.