I found a leak in the fitting that screws onto the tank selector. Over 1 1/2 year, it leaked out about 1/3 of the tank. Not dangerous but wasteful.
Soap suds have always worked for me but there are some sophisticated sniffers like some professionals use but don't know if they would do better than soap. I doubt it. If you can smell it, I would think soap suds would find it. Make sure you are getting the soap suds to every single part that's pressured. Valve stems are notorious for leak and my selector valve has a big fancy cover that you can't get the suds to without removing the cover. Too bad you can't submerge the whole thing in water like we used to check inner tubes for leaks, ha. You'll find it!
Rockwood 2104S, 2014 Ram 2500 Diesel.
USMC 68 -70