I take it the 'way under' valve leads to another knife valve at the side of the trailer. If so, you can leave it open (providing it doesn't catch on anything while on the road) if you want to. My theory on the 'two valves' system is to keep the weight off the pipes. By closing the one closest to the tank, the outlet pipes are empty. But in a campground, there's no reason to worry about the pipes having weight in them.
If on the other hand you're talking about the only knive valve in that run, you shouldn't leave it open. A black water tank has to have an accumulation of water/liquid to operate. Leaving a black water valve open all the time allows the liquid to drain off and leaves solids (you know what they are) to accumulate and harden. One does not want those solids to harden.
83 sweet sweet days camped in 2008 - not easy in my neck of the woods!
- Salem 401-2b; 31" Sony Trinitron and two channels on the antenna.
- Smokin' hot wife... 2 boys, 25 and 10.