We chucked our OEM mattress as soon as we got the trailer.  Having slept on original mattresses in other campers, I knew it would be miserable sleeping on it.  We use a 
Colman 4-n-1 airbed, as we deflate it in our collapsible TrailManor when we travel.  Simple to inflate.  HOWEVER, in looking for a link, I noticed there are very different designs of the same item.  ours has a zippered connector flap between the two units, so we don't slip between them. Some that I found for about the same price as what we paid, don't appear to have the same connector between them.  
But, we top it with a 
foam egg-crate topper, which makes it as comfortable as our Sleep-number bed at home.
Great bed for under $100.  We use a 12v inflator to pump it up when we arrive at camp.  You may not need to deflate between trips, but I bet it would lose air over a month or two.  I don't have to inflate after the first time when we camp for a week.
Good luck!  A very comfortable bed makes a world of difference.