more comments on options
The King mattress takes an extra 12" out of your maneuvering space in the bedroom. If you are used to a queen then you will find it easier to get around up there. With the king there is enough room between the bed and closet but the opposite side (next to the bathroom wall) you can just slide in and out.
The RV mattress is not as long as a standard mattress. Otherwise I would say just skip the upgrade and go buy a good residential mattress. It is possible to cut one down but it is not something I want to try. I don't care for the foam mattresses like tempurpedic but some do, those can be cut down. I would say the Serta upgraded mattress is fine but it certainly is not like what we have at home. That way we can still call it camping.