Completed my shower install.
In the original conversion there was a small sink with a shower wand. The PO removed all of that cabinetry but left the opening with the pex piping exposed. He also gave me the leftover pieces and extra vinyl trim to match the rest of the interior.
I decided to add an exterior shower box and extra long hose which gives me the option to shower inside or out depending on the season. I housed this in a cabinet which would also snugly fit the porta potty.
In order to leave the original pex fittings and insulation intact, I chose to run the piping directly into the cabinet rather than back through the wall. I drilled 5/8 holes to route the piping. It fits like a glove and keeps the piping secure.
The biggest chore was getting proper fittings with the pex crimper. It was particularly challenging in tight spaces. Once finished I was relieved to find no leaks and great pressure from the shower wand.