I just got a call from Eric Becker at Manchester Tanks in Elkhart, IN.
As it turns out, asking about RV LP tank valves is a complicated question. Here's a summary of what we discussed.
1. Some RV tanks will have a back seating valve, but you have to contact the valve manufacturer to find that out, most don't
2. All valves should be opened all the way or closed all the way, BUT:
Valves should only be opened and closed "Finger Tight".
Valves that leak at any point when partially or fully open should be serviced or replaced. Most RV tanks valves are not intended to use back seating to seal.
Partially open valve (i.e. 1/4 - 1/2 turn) can develop icing in the valve and can shut down.
Over tightening either way can damage the valve mechanism, seats and/or seals.
First responders expect valves to be fully open when responding and will turn them off if possible.
Open valves all the way, but do so "Gently"
If there are any stem leaks
at any point after opening, the valve should be serviced or replaced.