I doubt you will get any place with a dually and heavy camper that a front locker will make a difference. If you are picking up a front wheel off the ground enough to loose traction, than you will be in a real pickle when the rear gets to the same spot and leans the camper over a lot.
Biggest problem you have is articulation because with the heavy camper you will need to keep your sway bars engaged especially the rear.
All that said, if ARB sells a unit for a 9.25" axle, I would trust it. Have heard of air leak problems with the ARB, but never the locker failing unless being really trashed on by a heavy footed rock crawler. Problem isn't the wieght, but the torque of the diesel that will give the locker a fit.