That is a "air line compression fitting". The brass "band" is what does the sealing. Usually they are smooth (like a wedding ring).
My coach was built with the "Push-To-Connect" style fittings. These are great for assembly line work but not so good for coach owners (they always leak and are hard to impossible to get the line out of for replacement). When I come across a leaking PTC fitting, I replace it with a standard "Air Brake Compression fitting. I just cutoff the nylon air line at the fitting - as most of the time you cant get the old line out of the old PTC fitting.
The most common sizes on our motor coaches are 5/8", 1/2", 3/8" and 1/4". Your photo looks like the line is for the tank drain, so 1/4 inch air line (with 1/4 inch NPT threads going into the tank) is most likely what you have there.