[folivier]

Adrian you should only need to drain the "wet" tank in the back, the big one in front of the engine, and the "wet" tank nearest to the 110v compressor, (and the 12v compressor tank occasionally) on a regular basis. If you're doing that there should be no more water getting in the other tanks. Since you don't know how the previous owner maintained the air system go ahead and drain all of the tanks to be certain they are dry. After that just periodically drain the front and rear "wet" tanks and you should be good to go! Adding pull cords to these 2 make it a simple process.