Putting out slide on incline?

NO>>>NO>>>NO. Not at an angle. You will put stress on the slide.

I would follow the manufacturer's manual. If the manual isn't clear I would call the manufacturer. I have a full wall slide and my manual is probably different than yours in procedures for operating the slide. The manufacturer would know what damage has been encountered opening a full wall slide on their units. That said, I open my full wall slide (about 26 ft long) in any position of my MH. After it is open I operate my jacks. That is the instructions in my manual. I'm sure you'll get lots of suggestions but deferring to your manual is best procedure.

If you operate the slides while the RV is significantly out of level (front to back), you'll be putting side loads on the rooms which will increase friction and load on the drive train. This will have a detrimental effect on rails, gears and motor. I would not do it.

Jim

JMO but I'm personally not comfortable extending even our shallow slide or the awning unless the trailer is sitting relatively level. I too have a driveway which slopes noticeably to the street so I use an 18" stack under the tongue jack to minimize TJ extension and chock the trailer with a set of wheel pads that use the weight of the trailer bearing down on the pads to absolutely prevent it from inadvertently shifting. Pics are in my gallery.

No matter what the manual states, I wouldn't recommend opening the slide because of your extreme angle.

Every owners manual I've read has always recommended that the rig be level when the slide is used. I think I prefer to err on the side of caution.