The 3-way frig is not that big of a user of the propane. I've had a full tank of propane (20#) last a season with the frig and inside cooking stove on maybe four or five long week-end outings. The furnance is what will suck up the L/P in short order - the warmer you want it the more that furnance will use! In real cool weather (early spring/late fall), with temps dropping down into the 30's at night, a twenty pounder will be gone in a 4-day long weekend, and that's using it conservatively.

I ran through 2 20# a few years back over columbs day weekend up in the adirondacks. Granted DD #2 was just a baby, under 1, so we were running it real good, that last morning we ran out about 5am, made for a cold morning getting up and out. Should have filled the 1st empty when it ran out. Live and learn.