Just to be clear, 1/2" pipe thread won't connect to a garden hose, but it WILL connect to shower hose.
Only thing is I don't know of a quick connect that would be suitable.
What I would do is get another shower hose and head/nozzle, and to use it, just unscrew the original shower head, and screw on the second hose assembly using a 1/2" NPT brass male-male coupler.
Your water runs all through the camper with just 1/2" PEX tubing. I can't imagine an extra 6' of shower hose is going to cause THAT much head loss, especially if you are going to be using it outside the camper where it can gain a few feet of head due to gravity.