The RV mattresses are shorter than regular sizes no matter if queen or king. When we bought our current RV, the first night was so bad that I even considered sleeping on the carpet floor with the dogs as I'm sure they had a better night than my wife and I.
We went to Sam's and bought a memory foam king topper the next day. The fitted sheets scrunched it enough that we did not have to cut it. That's been over a year ago and we've had good nights sleep since.
The memory foam topper is the way to go. The hard-as-bricks mattress that comes with the RV gives good back support and the topper allows the bricks to be able to be slept on.