[I once asked a KC-10 boom operator how they did it (though it was probably KC-135).]
though it was probably KC-135s that did most, if not all, of the refuelings.
They guy I spoke with was definitely KC-10 crew as they were standing outside of their aircraft.
Boom operators, like pilots, can be rated in more than one type of aircraft.
I remember when the KC-10 came out.
I was going to the NCO Leadership school and my room mate was a KC-135 boom operator and wanted to get a KC-10 slot.
I never did learn if he did. All I know is he had several Air Medals (which are worth promotion points), one for landing the aircraft with only two engines, which, of course, he didn’t do because he wasn’t the pilots. I was a little ticked off that the USAF didn’t have a similar medal for us non-flying types.