A few years ago I went over to the storage lot to have lunch and work on the RV. When I got there I heard a pronounced hissing that turned out to be coming from the OPD of my neighbors very large horizontal tank. It was a warm and very still day and LOTS of RVs in the immediate area. I thought about what to do for a while and finally stuck a hose on the OPD pipe (never seen one like that) and taped the other end to the top of his ladder to vent it 12' in the air rather than directly under the coach, then called the fire department who did nothing further.
He had just filled it so it probably vented, but then the internal spring broke and there was no way to shut it off (he showed me the pieces afterward). That's a pretty scary thought if that were to happen in a campground.
