Where did you source the slides from?
other options...... do a box in a box (aka a slide out). Also like others mentioned... remove roll up (they are hard to insulate well) and do a door to a door... well in this case... door to a drop..... as in a rear wall with door or slideing window as another mentioned along with a drop down that becomes porch floor. Drop down easier to insulate....
Also been think about not removing the rear door ? Maybe a fold down patio ? Or screened in room ? Got the idea from a photo of someones truck with the owner and daughter in a shot looking out. Thanks JAVA.