In our Excella with a rear bedroom, we did a very simple conversion from twin to King by placing a plywood piece between the twin bed bases, supported on a small ledge, making a large platform across the back of the trailer. Then we strapped the 2 twin mattresses together and turned them sideways on the platform, added a king memory forum topper trimmed a bit, and use king sheets. With the curved Airstream twin mattress pushed into the end cap curve at a right angle there is some gap between the bed and rear wall for knee overhang and blankets, and space at the foot of the bed for storage. Disadvantages: One person has to climb over the other. You have to crawl under the platform to access the underbid storage, but you also get large open storage under the center, that we slide baskets, totes, etc into.
I added short sets of plastic storage drawers on the twin bed platforms between the bed and bath wall that fit under the Windows. Works well for us, and we can return it to twin layout whenever we want. We had to remove the nightstand between the twins, but it is stored in the basement for reinstallation.