Is the KC-135 landing gear life limited?
All landing gears have a limited lifespan unless they undertake in really, really big maintenance, as we saw with the Antonov 225, which went under maintenance for 18 months! And the KC-135 is no exception. Most landing gears last about 20 years, and the one's of the KC-135's is designed for 30 years. But even though it was designed for only 30 years, the operators of the KC-135 still extend it's life by a lot, with as you said, regular maintenance. So, you might see some KC-135 landing gears that are 40 years old!

Aging landing gear life extension
The committee is concerned that the Air Force has been extending the use of aircraft landing gear far beyond their original design life. The KC-135 was designed for a 30-year lifetime, but will retire at 86 years and the B-52, designed for a 30-year life, will retire at 94 years.

Source: congress.gov