We drove past a mattress store, found a queen mattress we liked and paid for it. Then we drove to their warehouse and in the parking lot, swapped the new mattress with the rock that came with the trailer and "donated" the rock to the warehouse employee who helped us get the new mattress into the trailer. Heads up: Putting a standard, house mattress into the trailer took a lot of force because they don't bend all that well. We got it in, but it was tough! We didn't think it would be that hard, honestly.
Bob and Beth
Husky Centerline WD
2014 Dodge Ram 2500 w/ 6.4 Hemi