Find a camper/RV place that sells parts. They usually have a catalog from a supplier that you can have. It is full of parts and pieces to do just about anything you want. To answer your question specifically, there is a self contained plastic trim box for the water inlet. You simply cut the aluminum sheeting and screw it in place. It will self trim to hide the screws. The outlet would be just underneath the trailer as it needs to be at the lowest point to drain properly.
I wish I could think of the midwest supply house that sells to many of the dealers. I have their catalog in my office. It's full of this stuff in the plumbing section. If you don't find what you need, shoot me a private message.
Rod
This link has an example -
https://www.rvpartsoutlet.com/newstore/nojava/showprod.cfm?ID=1631&refid=86A9E6E0%2DDC4D%2D4930%2D8CDEDE5085B91219
