I wish US makers could so something like the above. There are also European cassette toilets which swivel, so one can shove the bowl out of the way when showering.
I'm curious why US tank toilets are not more common, however if I were to guess, it probably would be that grey water is easy to deal with, while done right, hauling a cassette might be better for CG owners, as it means a lot less water into the sewage treatment system.
The downside of cassettes is that for one person, a 20 gallon tank can last a long time, while a cassette might last a day or two at most. However, one can always buy multiples of them, and put them in a hitch mounted cargo rack until one does find a place to dump them.