Big tank upside down, so *liquid* is transferred! It will work with it
right side up, but only if the big tank is warmer than the small one and
will take forever (the propane evaporates in the big tank, passes as gas
to the small, and condenses there).

Even when upside down, the big tank should be warmer than the small one,
so the vapor pressure there is greater and will force the liquid
through. It could drain by gravity, but the openings are so small that
it's impractical.

Note: "big one warmer" can mean heating the big one or cooling the small
one. And keeping it that way.

Getting the small one totally full can be done, but shouldn't. There
should be "head room" in the tank for expansion with temperature rise.
The standard is 20%, IIRC. If you wanted to be fussy, you could fill by
weight, but that would mean some back-and-forth to the scales.


HTH,
Bob