ARedmon,
I have an '03 QC, 4X4, NV-5600 & tow a 25' bumper pull travel trailer (7000 lbs). I live in AZ where we have some decent mountains. Phoenix is at the 1100' elev. & where I camp is at the 9200-9500' elev. (230 mile trip). That's an 8000+ foot climb.
So far, I have not found a NEED for an exhaust brake, however, I would like to have one (for fun).
Taking into consideration the type of terrain you will be driving & the fact that your fifth-wheel will be much taller than my conventional trailer (more wind drag on the FW), you may not need an exhaust brake. You may try towing without one & see how it works & then make your decision.
I'm sure there are folks that have the same set-up that you are interested in & tow in the same type of environment that you will be towing, that can give you better info. based on their experiences.
Another plus for the exhaust brake is that you can use it on a daily basis for slowing down the vehicle instead of using the service brakes. If used regularly, your service brakes should last almost forever.
If you don't mind spending the money, I don't see how you could go wrong by having an exhaust brake.
Joe F. (Buffalo)