In general, OEM RV mattresses are bad and easily replaced. We have replaced our mattress in every RV we've owned.
Obviously, a Murphy bed is a bit different animal. The size, thickness and weight all play a part in how a Murphy bed works.
You may or may not find others here that have replaced their mattress - if not, I'd talk to custom mattress shops or an custom online RV mattress company like
www.mattressinsider.com.
The easiest thing would be to add a memory foam topper - but with a Murphy that might not work at all.
Good luck and let us know how you fix the issue.