I think you are right, 15,000 is short miles for fluid and 30,000 for filter.
Reason I do @15,000 because that time I also replace air filter and PCV.
it's easy to do oil change every 5,000 and 15,000 trans. fluid PCV and air filter.
Hope that helps.
I went to local shop, He drained out differential fluid from rear drain plug.
from top, via rubber hose, through fill plug added trans. fluid til start drip.
I did check level on my regular trans., it's stay same, was before.