I did see someone on a youtube video who built in a bathtub under their bed in a van. They could access it by takingoff the mattress folding back the board that supports the mattress, the board was on a continuous hinge. I think the girlfriend insisted on having a bathtub. I doubt it got used unless they were staying in an RV park.
People get all kinds of ideas when they design their builds. Many of them will turn out to be wasted efforts and get removed later when they find out it just is not a practical idea for life on the road. A bathtub portable or built in is one such concept that just makes no sense.
Often times putting in a shower is also in that category. It can work in a large RV but is more difficult to do in a small van that does not have a large fresh water tank, no grey water tank and no water heater either.
But there are individuals who for their own personal preference want a daily hot shower. It is not something that is a needed or essential for the lifestyle but it is certainly something they strongly desire, enough so to overcome the obstacles of making it happen.