Correct. The SR-71 uses JP-7 fuel, and is the only aircraft in the fleet known to do so. So, the USAF has to have custom KC-135Q tankers just for SR-71s. They can't be used for any other fuel.

The thing that made it obvious to me that there was a replacement for the SR-71 was that when they retired the aircraft, they didn't retire any of the tankers. That meant there was now some other unknown aircraft flying that also used JP-7, and it must therefore be the replacement for the SR-71.

Note that JP-7 is so hard to light on fire that if you throw a match into a bucket filled with JP-7, the match will get snuffed out. You've got to be a lot hotter than a match to light that stuff.