Demco Stay In Play is an amazing system especially for extended braking on hills etc.  Other systems will time out and turn off the toad brake controller after so many seconds, or the inertia types may brake inadvertently when you are using your exhaust brake or other ways to slow down without braking.

I have a Discovery 39F and tow a Canyon with my Harley on it and use the RVIBrake 3 system. Works great and does not have problems with engine brake. Much quicker reaction time when braking is applied as the pedal actuator uses compressed air.

I has a command centre tablet that you mount on your dash that monitors the tow vehicle function. You can see the brakes being applied. 

This is a portable system that is easily transferred between vehicles,

I suggest adding this one to your list for consideration.

2017 Discovery 39F