I would look at adding a standard RV outdoor shower box on the exterior wall behind the kitchen counter or bathroom counter where there will be hot & cold PEX lines you can tap into. But the typical outdoor showers are cheap pieces of plastic sh*te tho.
I can't see a reason why not to use the low point drains but sounds like a lot of monkeying around to get the right fittings to be able to use a shower head and mixing valve and to make it removable for travelling. And it'd be more stuff to store away somewhere. You could just replace the existing drain valves with ones with hose fittings.
In the 5 seasons of using our current TT, we've used the outdoor shower exactly one time. :)