I've done it!
I did this exact conversion to my 34 footer.
I built a "bridge" out of 3/8" plywood that jumped from side to side. On the bottom of this sheet, I put 3/4" thick by 2" wide stiffeners that were about 1/8" shorter than the gap between the two existing twin bed platforms. These 3/4" strips not only stiffened the 3/8" plywood sheet, but they also hold it in place between the two existing platforms. I put them about every 10" down the length of the piece. So my King mattress is lengthwise direction from side to side in the back, and it's widthwise direction is from the back of the coach running forward. It works great.
If you can't get a good nights' sleep, then your vacation will be ruined. A good bunk is the #1 thing you need to have in a trailer.
Oh, I got those cheap foam pool floaties and wedged around the ends of the mattress against the back and sidewalls of the trailer to "lock" it in place. It's worked great.
Best of luck,