We replaced one bunk mattress in our 32QB with a 3" foam topper we bought from Overstocked.com . They have many different types and sizes. The prices vary also so you need to look at what they have. Shipping is only a few dollars and sometimes they offer that for free. We also bought one for our queen size bad, what an approvement that made.
My son who slept on the new mattress also said it was alot better then what came with the rig. I hope this helps you out.
2003 Excursion (his)
2000 Excursion (hers)
2006 Zinger 32QB