Thanks everyone. I called Keystone right after I posted. They gave me two numbers to call near our location (we were at a Loves RV Hookup in Eutaw, Alabama. Called this place...
https://countryroadsrv.com/
They had several replacement handle/ valves. It was a 45 minute drive in the direction we were going anyway.
One of the techs replaced it, took 20 minutes. $85 which I was happy to pay. Back on the road.
PS: watched a YouTube video
https://youtu.be/Ip2ib58bnyU?si=yQO3KsTD4V8t9dNX
and could have easily done it myself but didn't want to take a chance that I would break something else, like the plastic elbows etc.
Very happy this evening after a hot shower.
I do wish there was a way to isolate each of the water users. I think Riverstone has that feature.
Thanks again everyone!