2001 Kenworth T2000 air suspension supply issue

This is simple: Trace the line to see where it goes from the leveling valve. See if there is air going to the leveling valve. If not, follow the lines and see where it connects to next. You may have a blockage or collapsed line. Drain your air tanks and see what comes on. There are only so many air lines on the truck, worst case scenario tap right into the tanks.

 

I recall working on a truck with a similar problem, been a couple years ago but I believe it was a KW. Traced the airline for the leveling valve directly to the air dryer. Had a sticky valve in there, cleaning it up solved the problem.
These problems require patience and a willingness to crawl around underneath your truck, tracing wires and air hoses. If you're persistent, you may be able to not only solve this problem, but prevent other problems before they cost you time or money.