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So Wanderlodges have this set up on the LXi models. They use a Chelsea PTO mounted to the Allison.
I am a bit confused as you are talking about a ZF gearbox, but your signature says Allison 4000 series. 

What is your fan hydraulic motor set up? Would think it’s a pilot operated bypass on the fan motor that is controlled by an external controller which receives its commands from the ECM. Actually just read through here and looks like your fan is just controlled by a DC solenoid, not a PWM from the ECM? 
So when you are talking about fixed displacement pumps and fixed displacement motors it’s actually quite easy to set them up. If they are fixed displacement, let’s say the pump is 1 cu inch displacement and the motor is 1 cu inch displacement then that means they will each spin very close to the same RPM. So now you can start doing a ratio to get the fan speed where you want it to be, generally limitation being max fan RPM. The fan manufacturer should be able to provide you with their max RPM, but actually more important is their desired RPM, which is where the fan is most effective. Some fans actually become less effective past a certain RPM, or a diminishing return let’s say. 
I have done quite a bit of research along with another one of our Wanderlodge members and we have upgraded our fan systems to make them more effective. We both went with a PWM controlled system, that can vary the speed of the fan based on an input signal. We are using a fairly accurate and reliable GM coolant sensor that sends its signal to a custom brain box, the box then tells the fan controller what to do with the fan via the PWM signal. PWM by the way is pulse width modulation, which is a fancy way of saying they basically switch the DC voltage to create a square wave form signal.