We never even took the plastic off the OEM mattress. Moved our Sleep Number from the Montana to the Redwood the first thing. I think the Sleep Number might be a little longer, but being nothing but a fancy Air Mattress it compresses a little without a problem. We've been sleeping on it for almost all of the 2 years we've had our RW.
Michelle & Ann
2018 Chevy 3500HD Crew Cab High Country DRW, D/A, 2016 RW39MB, Dual ACs, Auto Level, Auto Sat Dish, Stack W/D, King Sleep #, 17.5" Sailuns w/Disc, MORryde IS & Pin, Comfort Ride Hitch, 5.5 Onan, Res Fridge & Induction Cook Top