Cassettes are a pain in the ass to install and fitting them to an RV sounds like a bigger pain. I installed a three zone ducted mini-split for a fellow a few years ago and he insisted on putting the outdoor unit in his small attached garage with his other outdoor mini-split. He opened the garage door and found lots of melted plastic his first summer.
There are already existing holes and mounting points, from the two roof units as well as Fantastic Fans, and the headliner/ceiling could stand replacing. Not too hard to take out and replace, looks doable to me as a project.