Braking efficiency varies from coach to coach. No good rule of thumb. Air brakes are usually very efficient but they also require more attentive maintenance. Modern RVs with air brakes have automatic slack adjusters that require little or no owner attention - but like anything else they can have problems.
Perhaps some of the descriptions above in the thread are a little over-generalized. For instance, not every RV diesel is delivered with a Jake brake. Some may have Exhaust brakes, and a few have transmission retarders. Some slightly older or smaller RVs may have hydraulic brakes rather than air brakes.
My understanding is that among properly maintained systems that the retarders are most efficient, followed by air brakes and/or Jake-style, and then exhaust brakes. I'm open to correction.
Whatever system you have, the best practice for descending a hill is to stay in the same gear you went up it with, or at least one gear lower. Don't ride the brakes on the descent. Do "stab" braking to keep your RPMS below the point that they would force an up-shift. Engage whatever extra braking system you have.
BTW, someone mentioned AutoPark. The guy I bought my coach from can be found in the technical area of the RVNet/CoachNet forum. He may be the most knowledgeable person in the country on those units and has a tremendous library of information, fixes, etc. His screen name is OldUsedBear. He is a great resource for those with that type of brake.