There should be a check valve between the water filter and the tank. If there is, many coaches, including mine, have an automatic electrical drain at the bottom of the water bowl. It empties the bowl every time the compressor shuts off. In the process it takes the head pressure off of the compressor which aids startup. The 12 volt automatic system is complicated and when it failed I replaced it with 120 volt system which is much simpler. A good bubble spray is found in a children's toy department at Walmart. Children us the stuff to blow bubbles. Just add a little water and you will be able to find the smallest of air leaks.
The blue hose in your photo obscures the bottom of the water bowl.
I went to Dollar Tree and for a buck got a 32 oz bottom of kids bubbles. I soaked everything on filter and lines to the tank really good and could not find any leaks. I said to myself, I may have to turn in my man-card if I don't find this :-) or at last my Guru card. However, I did find a leak on the regulator valve (or that is what I think it is - picture below). The leak is just to the left of the brass nut on what appears to be a joint. I can push around on it and stop the sound of the leak. Does that sound like it is cracked? Anyone have a part number?
What about instructions to replace?
Thanks Mike
Based on the knob, it looks like an air pressure regulator.
Sometimes just unscrewing a fitting, cleaning it, retaping with teflon (if an NPT pipe thread) can stop a leak.
Forest is correct. You may also try turning the left brass nut and see if the leak stops. I would place even money it will. If it still leaks you need some DOT 1/4" compression ferrules, remove the left nut, cut off the old ferrule, install a new one and tighten the nut.