Happened to us once in Kentucky at a KOA, on Lake Barkley as I recall. Guy pulled up with a Honey Wagon in tow, didn't say a word, just connected to the trailer's waste outlet and opened the waste gates. Sure was a surprise to me as no one said anything about this "service" when we checked in, it just "happened". :E Actually, I wasn't too pleased about it, some guy I'd never seen before fooling around with my trailer but by the time I came out of the trailer and figured out what he was up to he was already halfway through the process so I let it go. Annoying nonetheless. :M
