Following up for others' future reference. BlueBird90's instructions were right on. the lock ring was a little tough to get started with the punches we had, but some penetrating oil and a few tries did it. There was initially 1.125" clearance between the throwout bearing and the tranny input bearing housing. We adjusted it to .75" by threading the tube out of the collar at the release fingers and towards the rear. It has normal right hand threads and turned easily by hand. We threaded the external adjustment on the horizontal rod in as far as it would go, but it still releases about 3/4 of the way to the top of the pedal travel because the disk is so worn. Free pedal distance is good.
BB90, I'm agreeing that the clutch is pretty worn. I would have liked to send these folks on their way with everything checked out and fixed, but it's time for them to get out of our driveway. They're good at finding people to help them along the way. I drove the bus home about 80 miles last fall over some pretty steep hills and couldn't detect any slippage. I've also driven it around locally pretty aggressively to check it out and had no problems. I think that they've got a couple of thousand miles left on this if they treat it right.
I never did find an ID plate on the clutch housing, but did find a remanufacturer's tag on the pressure plate. It says "Like-Nu, Remanufactured by Midland Brake" Model number is 140-601-1854. Photo below.
Thanks for your help BlueBird90. This forum has been a big help with the bus. I'm going to eventually go back and make a thread consolidating what I found on registering/converting a bus to a motorhome in Washington for this project. With the applicable RCWs in hand, it's pretty hard for them to force you to get it inspected.