There are three ways to figure out how much propane your appliances are using:

1)  Get repair/maintenance manual from VW or RV dealer and look up.

2)  Take appliances apart as appropriate.  Check burner orifice diameters
with orifice guage.  Check pressure (Inches of W.C.) at regulator.  Using
Gas Engineer's Handbook, calculate or look up flow rates.

3)  Weigh propane tank.  Install in bus.  Light stove burners, run for 1
hour, turn off.  Weigh tank again.  Difference in weight = consumption rate
per hour.  Do same with refrigerator.  (Do refrigerator in hot weather as
the amount of propane burned goes up as the ambient  temperature goes up.)
This method is the most practical, effective, and accurate.

If you plan to camp in one place or away from propane sources for longer
than a couple of days, I'd suggest that you plumb in appropriate fittings so
that you can use an auxilliary propane tank to replace or supplement your
Westie internal tank.  If you set the regulator on the aux. tank just a
fraction of an inch W.C. over the westie tank regulator and turn both of
them on, the aux tank will empty first - then the Westie tank.  If you check
the aux tank a couple of times per day, you can pull it and take it to town
for a refill or replace it with another spare tank when it empties without
drawing down the internal tank too much.  

I used to do this when I camped a lot in the back woods in Wyoming with the
kids - and had to run the heater at night.  I've even seen some people who
go out in the woods for extended periods have the local propane dealer
deliver a large bulk propane tank to their campsite.     

Good luck,

leonard jones
currently a wannabus