Firemen don't do pre-trip inspections tho, they just jump in and go - they only care if the brakes work, not how or why they work, as far as a firefighter is concerned their trucks are just very very big passenger cars (well ones that carry lots of water too). With a commercial truck it's different, the sheer mileage you rack up on those things requires you to know how they work, especially since most of the time you are pretty much on your own when it comes to inspection and preventive maintenance - not like a fire truck that gets checked out by a professional mechanic on regular basis.
The RV motorhomes tho, those I agree should need an air brakes endorsement, cause they can accumulate mileage and therefore wear like a commercial truck, but unlike a fire truck they don't have mandatory periodic inspections. They also sit a lot, which IMHO is just as bad as having the wheels driven off them...