"...I think a better solution might be the stick-on
liquid crystal thermometers used for barbque
tanks "

Thanks.  I had no idea those existed.  For a relatively low-tech guy
like me, that's a much more appealing first shot at a solution.

Only problem I foresee is that those stick-ons are calibrated for
tanks whose distribution of propane within their volume  would be more
like a **vertically mounted** Westy tank.  It will be interesting to
see how sensitive/meaningful its readings will be to drops in the
shallower Westy tank.