I would think if manufacturers had meant for the slides to be supported, they would have included them in the sell so you could pay more. Lot's of pro's and con's on this...ie, if you put supports under the slide and if slide is on the side where a tire "might" go flat, then you're pushing upward on the slide, force that wasn't meant to be there...saw that first hand in a campground where it looked like campers had rented a space for a season (?), no one there, three slides, all with supports underneath them and a flat tire, one slide was being pushed upward in an unnatural looking position due to the tire being flat...tire in front appeared to be ok, but low, it HAD to have done some damage to the slide.
I know there are quite a few manufacturers out there that DON'T recommend slide supports. Your very best bet would be to call the manufacturer and get your answer, "straight from the horses mouth" just to be safe....only cost you a phone call...well worth it to make sure you're doing the right thing and not possibly damaging your 5er's slide unit's