I just returned from a week and a half long stay in the Arizona mountains. I had full hookups and everything was working fine until I tried to drain the grey water 1 tank. It simply wouldn't drain. I pulled the lever and nothing. After pondering for while, I hooked up the water source to the black tank flush system and flushed out the black tank (grey water valves closed, of course). After that, the grey water 1 tank drained just fine (grey water 2 never had a problem draining). Maybe it was a fluke, so I tried it again a few days later and the same thing. It might be from the altitude (6500 - 7000 ft.). I did this three or four times on the trip with the same results. It didn't seem to matter how full the tank was. Has anyone else ever had this problem?