AirNav (and FAA data) doesn't have a lot of Sheppard's frequencies. Just the main ATC ones.
The db for Sheppard is all messed up. Look at the alpha tags. Most of them are all the same. There is non-aviation stuff mixed in with the VHF/UHF AM aircraft stuff. Some of the frequencies might be obsolete by now. I haven't checked it in years.
There is more unconfirmed frequencies in the wiki and a link to a USAF order with most of the 20-channel presets.
wiki.radioreference.com
I think there is a pool of frequencies used by Sheppard, Fort Sill, NASFWJRB and others for the various training routes around here. I think the routes have their own frequencies that are assigned on a daily basis to whoever is flying, like the ones for the refueling tracks but I was never able to figure it out.
I think each squadron at Sheppard might have their own aviation frequencies (also TRS talkgroups) but I was never able to figure that out either.