Paul,
Yes you can. The brass colored knob is just pushed on the stem. Check the orientation dot before you remove it off the stem. Remove the broken serrated outer ring off of the valve. The name of the plumbing fittings you are dealing with are called "Flair it". You can purchase a 3 way valve #16910 from an rv dealer or amazon. Remove the "T" handle and toss it. Remove the serrated knob and pull out the stem assembly. Install the stem and knob on your existing fitting in the manifold. Install your brass knob and perform a leak check.
My coach had about 5 of these leaking when I bought it because it was allowed to freeze. Inspect all serrated rings for evidence of cracking. Don't forget to check the ones on your Hurricane water heater if you have one. Fred