I haven't done it, but I have thought about it.
I believe, an rv king mattress is the same length, and 12" wider (72×80 vs 60x80) so you should be able to fit one by removing the rear nightstand and building a platform to support it between the current platform and back wall.
That should be pretty basic.
I have no idea what shape the walls would be behind the wardrobe cabinet and nightstand once removed or if you can order a small cabinet from FR to fill the void left if you pull the rear wardrobe cabinet.
I am interested to see pictures, if you start pulling it apart.