2000 freightliner mystery no drive scenario

I have a 2000 freightliner Fl112 car hauler with the cat c12 and eaton 13 speed. 3 weeks ago, I was driving it as normal when I was driving it wouldn't shift into 13th suddenly, it would just remain in neutral. I kept it in 12th, every so often trying to put it into 13th. Eventually, it went into 13th and drove that way for about 2 miles before it dropped to neutral again and I reselected 12th and kept moving. About a mile later, it dropped off completely and I ended up coasting to a stop. The truck was running fine but through the whole range, I could not get a gear. The strange thing with the clutch depressed and you select a gear, theres a tug as if the gears are connecting, but then nothing and still no forward momentum. The pto gear worked with no issue. There is an air leak at the module on the top drivers side of the transmission, though I don't know if it's actually a leak. The shifter range and gear select seem to work just fine when used, but no forward momentum.I had the truck towed to the local freightliner whom tested the vehicle and reported back no driving issue. Then they tested it again and said 10-13 didn't work. They were too expensive to even look at the truck so I went and drive it home from Jacksonville to tampa, or about 240 miles, avoiding 10-13. Stopped for lunch, everything worked fine up until I pulled into our storage lot where it dropped off again. Exact same issue. Had it towed locally and when they tested it, once again no issues to report. I'm going to go pick it up tomorrow and see of we can possibly see anything, or get it to repeat. The mechanic can't. Diagnose without a symptom and I'm ata loss. If anybody has a general guideline or advice how to try to get it to act up again, or what it could be, any help appreciated. This truck has been down far too long. Thanks for any help in advance. I will get model of transmission as soon as I get the vehicle