Hi Bob - can't help with install advice, but here's our suggestion: you can eliminate a lot of modification work by just using a Coleman hand-held, battery-operated shower and a large kettle of warm H20.
13 has no shower; if we are boondocking
, we set up a 'privacy cabana', heat the water, step into the bottom section of the 'shower stuff' storage tub and suds up. FYI Note: the frame of our cabana will NOT support the weight
of a solar
shower bag). If you are just rinsing off in your swimsuit after a day at the beach/lake/etc, don't even need the cabana. When we get ready to travel, everything stows neatly in the storage tub which rides in it's assigned spot in the tug. Doesn't get much easier - we've used this method happily for over 3 years. However, YMMV.
Happy camping - L 'n D