Here's a picture for you.
from La Casita Bus (around picture #152)
I totally feel you, my wife says the same thing and I have scratched my head a lot trying to figure out a way to do it realistically. Logistically it's just not going to happen in our modest 30ft bus. Not just for space but because I don't think it is practical for my wife to take a 70 gallon bath out of our 60 gallon water tank on daily to weekly basis, I don't want to spend a thousand dollars+ on a decent on demand water heater, nor do I want to ensure I have an electrical system able to support such high wattage (though I think there are propane heaters). I also do not want that to drain into our grey water tank afterward. It's just not going to happen in this current conversion. I am not heartless though, I will get her a gym membership somewhere that has baths or something like that.
I am eventually going to build her a beast bathhouse out of a trailer after I finish my conversion. This will be specifically for the purpose of her taking a nice bath. I would eventually like to put a garage door on the bathhouse so if we're somewhere cool she can enjoy the scenery out of the bath. I don't know if I will use a conventional bath, but I have heard of people making baths with chest freezers due to their high insulating properties, and I like that idea. My Japanese wife definitely will need a highly insulated bath so she can take a nice long bath (which gives me more time to get work done). The bathhouse itself will also be insulated very heavily so she can have a sauna to roll around in after taking a bath. If this bath needs a 3000 watt space heater, I will get a generator to burn as much gas as necessary for my wifes comfort however this is all a second priority to the bus conversion, first we must move into the bus.
I believe you can do it, however I am one bitter Betsy about the idea of a bus.