[rluhr]

Our experience has supported that theory about problems resulting when the gray valve is left open. For two years we closed the gray valve and only used it for dumping. This left the pipe pretty clean after dumping.

Then we stopped for three months and lived in the Airstream with the gray valve mostly left open. A disgusting slime mat built up in the dump pipe and hose. (The slime mat is a layer of bacteria that is feeding on organic material.)

We have been using enzyme-based tank chemicals to encourage the breakdown of this mat, and have resumed our practice of closing the gray valve except to dump, but progress has been slow. After a month of use, the pipe is just now coming clean again.

There's no question in my mind that prevention is far better than cure in this case. I agree with the advice to leave the gray valve closed rather than letting it run for long periods of time, and to keep some chemical/water mix in the tanks to help organic material break down inside them when the trailer is stored.