Officially, other than mercs/volunteers serving in the Ukie Army, there are no Americans there.
Unofficially... well, we all know what the CIA and such do.
Now, that said, ostensibly this equipment came with crew training in the US after the Russians blew the crap out of several training areas that had been set up in Ukraine and there were no advisors or contractors in the combat area. The vehicles aren’t even being repaired over there, but they’re being shipped back to the US - and the damaged vehicles have been seen and photographed here.
(Even U.S Soldiers can’t maintain a Patriot system in combat without civilian contractor support....)