I think the limitation of 20 nodes and 9600 baud is related to the huge caps on the A/B lines.
Trucks have a very noisy electrical environment that would benefit from the reduced slew rate.

If you are using the diode termination method (no caps) you should be able to get much high baud rates and many more nodes.
I've run DMX at 250K baud with the diode method. But of course that was a 1 master, many slave arrangement.

I've never used the full J1708 protocol ... just the arbitration process on a system that only had a max of 5 master nodes.