[quote author=fishon71].... if I add 6" to the box I will have less than a foot to slide thru, what do you think?[/quote]
Whatever you're comfortable with. When our coach is in storage and I need to get past the bed, I'll sometimes retract the slideout a little, just so there's a foot or less between the foot of the bed and the cabinets. OTOH if I need to raise the bed to get to the stuff stored under there, I need a lot more room than a foot for my butt to get in there while I'm bending.
Lucky you having a spare air mattress. BTW the company still uses the Select Comfort name, but uses Sleep Number for brand marketing. I vaguely recall seeing another company with a similar name, which might be why they changed the brand name. Take this with a grain of salt though.