We found a 3" king memory foam topper at at Big Lots with a cover included for only $120. It's every bit as comfortable as the more expensive one on our bed at home! Don't know if they are still available. It was a little too wide and long so I figured I'd have to trim it, but the sheets and even the form-fitted bed cover that came with our Hurricane fit fine so we didn't trim it. The only place we found for anywhere near that price was overstock.com. Glad we didn't have to order on line sight unseen. Good Luck !!
Happy RVing!!
Donnie & Becky