Slides on truck campers

I think the intent is leveling the camper before running out the slides is the preferred approach to avoid possible drive issues.

Can you extend the slides without leveling, sure. Might it cause uneven wear or jamming, possibly. Can you drive with a slide out, apparently so :-)

Grit dog wrote:

That seems strange. Means every time you stop for lunch or a quick sleep you have to run the legs down to the ground to run the slide out?

Well no, our camper is fully usable with the slide in, don't deploy for lunch/quick stops. Been a few overnighters where I only dropped jacks on slide side to run it out. Have never deployed slide with jacks up.
As jimh425 alluded to ours is also a 2001- slide mech not so much the issue, the large full wall square hole is. Structure isn't what might be found in newer units. Also easily racked.
I wouldn't hesitate to deploy our slide on flat slope but my point, as I gather OP new to T/Cs and possibly an older unit, was only a caution on using slide regardless of slope. If OP has manual and it states putting jacks down I would heed.

^ Agreed and good word of caution. Just never occurred to me that they would be a limitation.

I heard the recommendation for jacks down before slide operation applied to motorhomes, where body transfer twist between the axles.
I deployed slides on my campers hundreds of times with jacks up.
Both my campers had similar build with metal slide tracks pushed by gears.
The upper slide corners have about 1" clearance to the wall and that is what can close with uneven jacks extension.