Added an exterior spray port / shower today. Used the Phoenix unit found here: 

Phoenix (SA-HC-BTW-RT) Spray-Port Outlet Box

​This was done on the street side of trailer between the side marker light at the rear corner and the main electrical connection point. Picked the Phoenix unit because the opening required is only 5.5" square, has hot and cold, and the hose connection is a quick disconnect. The unit is small because the hose is removed and does not store inside the enclosure. The electrical connection mod was already done with marine plug and removable cord that freed up space under the rear dinette bench. On the Scamp 16 with side dinette and front bathroom the hot and cold water piping runs through the rear street side dinette bench area along wall. Anchors were removed and piping was flexed gently away from the wall. Sharkbite tee fittings were used and 90 degree fittings on back of shower unit (threaded on one end, Sharkbite on the other).

www.homedepot.com/p/SharkBite-1-2-in-Brass-Push-to-Connect-Tee-U362LFA/2...

Small sections of PEX tubing were used to make the connections. Used butyl tape around the unit flange and stainless steel screws directly into the fiberglass. There are 11 screws so a wood backing was unnecessary. Need some basic plumbing skills and confidence to locate and cut the opening properly, but with the Sharkbite fittings it's not too hard. If anyone wants any more info on the install let me know. I'll try to post some pictures when I have a chance.