Here is a YouTube video on installing a Hepvo valve on a Casita shower - https://www.youtube....h?v=8tAe5s2KpqA. I have not watched it so I can't speak to how thorough it is, but if you don't get your questions answered, you can search for "hepvo valve installation" to return lots of hits. I did my installation several years ago in the parking lot of a public storage facility and there is not much to it. Watch a few instruction videos, read the instructions, take your time, and you will be golden! Best of luck.
I ordered the kit and replaced our p-trap and valve. The instructions in the kit and parts are all that is needed except some PVC cleaner and PVC cement. You will also need a way to cut the old parts out. I used a nail saw to cut the PVC. A Saws-all would be a bit easier and faster.
I suggest getting the front of the trailer up as high as convenient to allow one to work under the trailer. Easy to remove the original valve and p-trap and install the new fittings and Hepvo valve. Ours is a 1999 17' SD. Really a very nice change. Specially if you've ever found yourself wet, naked and in ankle deep water in the shower. Cause one forgot to open the outside valve.