I purchased an add-a room for a 25' trailer back in 1994. It created a lot of welcomed spaced.
AS the length of the rigs increased, I purchased additional door panels, using them together or on each end.
Currenly ,using on a 21 foot awninig with 3 door panels. In addition , just have the skirt that comes with add-a-room sewn on bottom of screen panels or find a dealer that replaces awning fabrics and use that when you grt a higher rig. The room never gets obsolete.
Hope this info helps======AJBJRVERS