Jayco Simmons Residential Mattress

We've been living in our new Jayco HT 28.5 RSTS for a month now, and, for the most part, love it. One concern we have is the Simmons Residential Mattress--it's hard as a rock. We have put a memory foam pad on top, and it's liveable, but after three weeks we find the mattress (not the memory foam) is slightly saggy in the area where I sleep. (Husband is claustrophobic so sleeps on the pull-out couch bed.) I weigh less than 160 pounds, so it's not excessive weight. Has anyone else had this problem with this mattress? Our last Jayco Superlite had an excellent mattress which we used for 8 years without a problem.