I just recently did the x-tend-a-stay conversion on my 84 westy. I
took the van up to the catskills last weekend and hooked up the barbie
tank outside the van to run the new propex heater. Temps outside were
about 12 degrees. The heater worked fine until the gas in the tank
reached the outside temp. About 1 hr. Thought it was the battery, tested
it ,bat. good. Thought it was frozen, nothing frozen. Decided to turn
that tank off and turn on the westy tank, worked fine all night. Temps
the following night in the 20's. Put the barbie tank back on worked fine
all night long. It has something with the surface area of the liquid
boiling off into a gas. A tank standing up that is 6 in  diameter and 30
in long has a 6 in circle of gas exposed. Take that same tank and lay it
down, you now have 6 in by 30 in of gas exposed. The more liquid gas
exposed the more gas it produces.I know it sounds hard to understand but
this was the way it was explained to me.
  By-the-way the extend-a-stay tee works great. I can hook up the barbie
tank or in an emergency i use the westy tank and carry a 1 lb bottle
just in case i run out. I made an adapter to go on to the 1 in. threaded
male that comes with the extend-a-stay adaptor.This adapter consists of
2 female post adapters 1 close nipple and 1 coupling all in 1/4 in npt.
If your westy tank runs out in the middle of dinner just break out the
adapter and a 1 lb. bottle. Twist it on and your back in business.