What are the rules for using this unit when towing a 5th Wheel-in flat country/uphill/downhill?

 

I'm not sure what you are asking here?
The GV will automatically disengage itself (go to direct) if you slow to less than 20-25 mph. They also state that it should not be used in OD for compression braking in low gear, because its pump can't develop enough pressure to hold it in gear unless the vehicle speed is enough to allow the pump to work.
Other than that, the trick is to split gears as needed to get you up the hills and across the flats in the optimum torque range for your engine. Great units!
If this isn't what you were asking, feel free to take another shot?!?
Steve

 

I have a four speed(overdrive). I have been taught the manual steps, but was looking for experience confirmation that it really works. I have found another site that confirms what your also experienced. Thanks.