Space is a premium in a motorhome. You’d like to have as much as you can but need to compromise to fit within the floorplan. For all the time you spend in the small room, there is little reason to make it larger at the expense of living space. Wet rooms provide more space than a cubicle with shower screens, so long as they remain water-tight. Maybe that’s why there are so few. An alternative would be to forego the shower and use site facilities instead. Some campers don’t even have a toilet.