We replaced our eastern king mattress with a residential high quality mattress that we purchased at Mattress Discounters. They delivered and set it up in the 5th wheel. DH said it was a bit of a struggle getting it in but they did it and hauled the old one away for us.
If that won't work for you, try a foam mattress that comes compressed and rolled up...easy to get into the RV then just take off the plastic and watch it expand. We have one in our house and it's incredibly comfortable and has held up well.