I can’t even imagine these newer generation idiots trying to steal a truck with a 15 speed tranny, let alone an old 5x9 dinosaur!
Heck! Even an old 10 speed Spicer would have them drooling with frustration!
I hold a class A (commercial) license. I've had to teach recent truck school graduates how to shift. One had gone to school which used lighter truck having synchronized 6-speed gearboxes. Unsynchronized Eaton-Fuller transmissions are quite different.
The first thing I had to do was keep the guy from burying the clutch to the floor. That's a big no-no when rolling, for it can rip the ears right off the clutch brake. With the big rigs, the clutch is ONLY pushed all the way when at a full stop --- and that clutch brake is there at the bottom of the clutch pedal throw, to help one side (or shaft) of the transmission to stop turning, so one can get it into a low gear. The rest of the time, if using the clutch, it's typically, if adjusted right, pressed about halfway through it's "throw". It took me two days, ending in some angry yelling on my part to break the guy of that habit (burying the clutch) He finally learned how to shift, and I eventually taught him how to drive in mountains (and not die). The school didn't teach him how to fill out a logbook either, but that's another part of the story of that particular trainee.
Running through the gears on a 10-speed (I like 13 speeds a LOT better) typically, without much thinking about it, I would end up single-clutching the lower gears, semi-float, then fully float through the rest of the middling gears using no clutch at all, then soft-tap double clutch at the top two or three gears. There are rational reasons for all of it. That said, the clutch will NOT help one shift gears in a big truck. The road speed, engine rpm, and selected gear all need to match. The clutch, if engaged and held there (engaged) will make it IMPOSSIBLE to get the thing into the next gear, for it will slow one of the gearbox's internal shafts. The input & output shafts need to correlate to all the rest (as mentioned).
It's easier to teach someone how to float, than it is proper double-clutching, yet a driver really needs to know how to do it all --- single clutch, semi-float, float, and double-clutch. Schools vacillate between teaching "float" or "double-clutch" and trainers vary. Many trucking schools associated with trucking companies having large OTR fleets don't provide enough road-time, and patient trainer (personality-problem free) driving time, before giving trainees actual driving tests, then putting them with a (typically lease-operator) driver trainer for 4-6 weeks of co-driving.
The lease operator gets paid for the all miles the truck travels, while the trainee typically gets paid a low weekly salary, and the company gets a "team" they can press into driving 20 or more hours a day, all of which is behind the economics of the "training" companies, among the large trucking fleets. Many loads have too far to go, in too short a time, for a single driver to legally deliver it, not breaking hours-of-service rules. Driving "teams" can otherwise be hard to come by. I much prefer driving solo, even across country, though running produce from the West Coast to say--- Boston, can be tough to make on time, and not risk log-book violation. Too much can go wrong on the loading end of things, burning up too many hours on 3 to 7 different "pick-up" locations, some needing have an appointment scheduled, some first-come, first serve (and they can run out of the right produce, too).