We have a new Sundance 310QB and of course it has that so-called mattress which is just a hard piece of foam about 3 - 4 inches thick. Does anyone have a suggestion for a mattress or mattress topper to put on the one we have? I would like to stay under $200. It's the Queen size. I've heard the 4 inch foam mattress toppers that Wal-Mart and others sale do good, but don't know.