Welcome to the forum!
The valve with the red handle on the left is for outside water, just like your hose bib at home. The device that has been added to this hose bib is a back flow preventer and/or anti-siphon device, so water can't get back into your trailer's fresh water system from your hose.
The capped water connection on the right looks like the flush system for the black tank that was on my 1999 Excella. If it is not your fresh water inlet to the trailer, and you have another place where fresh water hooks up, then my guess is it is the black tank flush system. (not sure if you actually have a flush system but it looks similar to my prior trailer)