Choice of mattress is a very personal thing - what is comfortable to me may be agony for you.Unless your coach has an oddball mattress size, go to a mattress store and try out various types , firmness and support.
If you have a non-standard size, aka "RV Queen" or whatever, your choices will be more limited but there are still options. Several companies will custom make whatever size you want and ship to your door, a "mattress in a box". We bought our last home mattress that way and love it.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL