I agree, no-sink is a configuration they should offer. However, I expect there are buyers who will insist on a sink that has privacy for various ablutions and thus they have it. Another possible design would be a small fold-out sink above the toilet or in the shower. You still need cabinets of course, which may be another reason for the sink.

As for recycling shower, to put it indelicately, when people wash their rear ends, there is matter flushed out that can't be recycled back for washing unless you have very fancy filters and sanitization. On the other hand a gray water reclamation system which takes the gray water, filters it, and loads it up with chlorine, then gives the chlorine time to percolate out to safe levels for human use (non-potable) could work. And when you drain out this water at a dump station, you could do it over the other side of the filter to clean that. Still, might be other toxins to worry about on the body (wash up after working on engine?).

Of course one simple approach would be to use waste heat from the generator system to distill gray water, if you're running that enough.
Then of course you get back potable water. Interesting way to watercool an engine.