I have a 2007 Coachman Captiva 265EX which has the same layout as an Outback with the slide out midget queen bed in the back of the trailer. Mine is 75" wide... a whole 3/4" wider than yours. I am 6'3", I can sleep in the tiny Queen but prefer more room. I bought a 4" memory foam King which is 75" wide (For some reason is it 1" narrower than an inner spring King).
I plan on putting a piece of 3/8" birch plywood 74 3/4" wide in the bottom of the "slide out" that I can slide in when the "slide out" is slid out allowing me to use the full length of the King. The plywood will slide into the living area of the trailer 20" blocking off one side of the dinette when it is slid into the trailer. I plan on having a 75" x ~20" piece of plywood hinged at the back of the "slide in" plywood. So when the "slide in" plywood is pushed all the way into the trailer slide out the memory foam mattress will bend at the back making a back rest at the back of the trailer slide out.
It sounds a lot more complicated than it actually is. I am currently making the slides for under the plywood. I was going to use some heavy duty bottom mount drawer slides but they were actually too thin to clear some of the slide hardware from the trailer slide out. Instead I am making my own "T" tracks out of some hardwood.
Anyway my plan is for a King in the rear with a side by side sleeping arrangement instead of the current crawl over mini-queen bed sleeping arrangement. In the evenings when we are watching TV or what ever we will leave the King Mattress fully in the "slide out" so someone can sit in the dinette and the couch isn't partially blocked. Then we will pull the King mattress into the living area of the trailer when it is time for bed... or atleast time for the kids to go to bed.
2006 Duramax Crew Cab Long Bed pickup.
2007 Coachman Captiva 265EX trailer.