I had to do the job while on the road, returning from Alaska trip. I was lucky enough that the public dump station we found at Smythers BC actually was designed for such emergencies (it would seem). It was a pit that I parked over to remove the old valve and it captured all liquids easily when I loosened the connections. After liquids were drained it was pretty easy to finish removing the old valve and replacing it with a new one. Hardest thing was to then fill the black tank with liquid again and drive the trailer around to mix it up before finishing the dump normally. I had anticipated a HORRIBLE job but it really wasn't that bad WITH the right facilities.
Good luck / Skip
