To answer your question, no the mattress is not attached to the couch. It is stored separately.
Since you bought the coach used, per your post, the previous owner may not have included them with the coach at trade in or the dealer may have misplaced them during cleanup.
If you bought from a Thor dealer they may be able to help but if you bought from a non Thor dealer maybe not.
One of the Axis owners might be able to give you a part number off the box or dimensions for the mattress.
US Army (Ret)
2020 Entegra Accolade 37TS
2019 Jeep Grand Cherokee Trailhawk (Toad)
FMCA - F432054