In my experience chasing leaks it is seldom fittings and usually devices such as valves. Both of my coaches had leaks at the belt tensioner control valve when I purchased them. The new coach also had a leak at tag axle valve, this leak was very small and not always present.
I found the best method was a stethoscope which is easily obtained at a tool store, I extended the hose on mine with a piece of tube and removed the amplifier device from the end so that it is strictly hose. You definitely know when you are close to a leak.