As Chuck said, that's the fresh water tank drain. It is NOT the OEM "capped tube" but apparently someone has replaced it with a "twist valve" drain.
You can buy a replacement (without the broken handle) at Amazon or at most RV parts stores. Here's the link for Amazon:
https://www.amazon.com/Camco-22243-D.../dp/B0006JLSN8
If I were you, I'd not install a new valve the way that one is installed. Access to the handle is the reason it's broken. You might want to unscrew that valve, look at the fitting on the bottom of your fresh water tank and think about going to Lowe's or Home Depot, in the plumbing department, and buy a nylon fitting and a short piece of PEX tubing and a "sharkbite valve" or a "sharkbite cap" and return it to "something like OEM"....
Here's a photo of the valve that's on your trailer. As you can see, the "twist valve handle" is broken. It can still be opened/closed by using pliers to catch the part that's left and twisting it to open/close the valve. My guess is that it won't last long if you do that, so I'd look at alternatives to repair it.