It's hard to tell just by a picture. Is the ground underneath perfectly level? There's no way it would slide out properly or at all if it was sagging 1or 2". Shouldn't be that hard to get the sidewall of the camper perfectly level then check the back wall of the slide. If the whole slide was sagging that much the window should show it, it doesn't.
You may just be dealing with an optical illusion... the fold out arm that carries the wiring slopes upward making it look like the back of the slide is low.
I predict first time owner panic, I wouldn't even sit in my slide out for the first couple of trips, I was convinced it would fall out of the camper.