You need to be very careful with propane, even small bottles.

Liquid propane will boil at a very low temp I believe its around 45 degrees
below zero. At that temp you have no pressure.

As the temp increases the pressure increases, on an 80 degree day you have
close to 200psi in the tank and the regulator drops it to about 1 psi.

A small Coleman tank will have the same pressure at the same temp so you
cant use a "regulated" device set for 1 psi with bottle or tank pressure it
will blow your lines and valves right out and cause a leak.

Running high side pressure inside the van either by having the tank or
bottle itself IN the van is dangerous because even a micro leak on a high
pressure can fill the van with propane in seconds.

VW and westfalia never piped in "unregulated pressure" into the van.

Be careful with Coleman bottles and storing them INSIDE the van, its not a
good idea. A better way would be in a heavy duty bag up on the luggage rack
or in outside storage somehow.

I am not sure how many westy tanks have the POL threads on the filler valve,
mine does.
A friend of mine just made up his adapter line to allow him to screw on a
bottle and run it directly into the filler POL threads.

All of this is OUTSIDE the van and much safer if there were a leak of any
kind.

A bottle connected to the tank is close to the same pressure and will
equalize. This is a very simply way to back up the tank on a trip where you
may not know the amount left in the tank.

One way to know would be if you could weigh the tank but thats difficult on
the westy. You would need special fastners with a spring load like a bbq
has.

I tried the tempature tape but the tank is too thick to show much of a
difference for where the fluid level is.

Too bad Manchester does not install or retro fit a guage.