[VAL] LP valve question

[Chris Koehn <timberguides@xxxxxxxxx>]

Hello list-
I have a question regarding LP tanks our 1979 Int'l Sovereign. We have 
2 forty pound tanks, converted to the OPD valves this summer. We used 
to have gages before the OPD valves stole those holes. Now we have no 
way of knowing for sure when we're running low on gas  until we run 
out. Interestingly that always seems to happen in the middle of the 
night...
So my question is- will LP retailers fill a partially filled tank, and 
charge for the amount pumped only? Most of the ones I've run in to 
charge by the tank size (assuming, I suppose, that the tank is empty).
I doubt that moving the switching valve to some position between the 
two tanks would allow one to empty completely without the other 
emptying as well..?

Thanks for the advice!

Chris K.
WBCCI 8638