Pardon Me for entering the hallowed ether of D3/4/RRS but...
I added cabin controlled elec. braking to our camper as, even though not a legal requirement, I can lessen the effect of the trailer trying to push the rear of the D1 around on extreme descents. It is also less strain on the D1 brakes all round & for the money, I'm more than happy with it & feel a lot more secure with the rig overall.
Steve

