Accessory regulators and connection tubing can be purchased at YFLPGS.  When
you attach the regulator to a standard LPG tank you have a valve controlled
(on the tank) regulated (11"H20) system you can connect to your propane
system downstream of the existing regulator.  You'll notice a manifold
(distribution block) into which your two propane tubes connect.  If you
relocate that junction by adding a second distribution block you will now
have three ports into the system, two that serve the refrigerator and stove
and one unused.  If you provide a fitting that caps that third port with a
standard nipple and cap you can attach the auxiliary system to that port
after the other tank runs out.  It is best to turn off the westy tank before
hooking up the auxiliary and leave it off while you operate on the auxiliary
tank.  It is wise to not drive with the auxiliary tank banging along the
road behind you, so doona forget to take the tank off before moving on lest
you be cursed with bad karma in your next life.  This will NOT refill the
westy tank, that is an exercise for the reader to imagine using a set of
clever adapters and wise use of propane resources.

P