There are 3 lines on shifter knob,Red,Black and Blue. All 3 lines go to the transmission. There is only one transmission gear box on this truck. The air lines all are hooked to a shifter mechanism,one is at the front of the transmission the other is at the rear,what I take to be the supply air line is on right rear center of transmission. There are several larger hoses on this transmission. I think there might be a gear pump to supply oil to the shaft bearings in this tranny.
If this was only a 10 speed I don't think there would be a need for 3 air lines. I have never been in one of these transmissions. It looks to me like it is standard 5 speed shift pattern with a reduction on the input shaft and a reduction on the out put shaft. The lowest gears would be the input reduction engaged as well as the out put reduction. Mid range would be the input shaft reduction in the direct position and the out put in reduction.
High speed would have input direct as well as out put direct. For some reason the chart in cab says do not split shift in lower gears.
I suspect that the transmission was let get low on oil and the input reduction shaft got scarred. The rough shaft might not let the gear slide on it to engage reduction. No problem seeing transmission,this is a cab over. You can walk right up to it!
I may need to install an elevator to be able to get in the cab. My knees don't take the climb too well.