How long does an average 20 LP tank last?

There's no good answer to that.  A few nights... a few months.  Somewhere in between usually.  If you have shore power, use a $20 electric heater from some place like Walmart though.

TT

If you are using the furnace in sub-freezing temperatures and running the refrigerator and running the stove and water heater (as needed), you may only get a couple of days out of a tank. If you are just running the refrigerator and occasionally the stove, a tank will last months. Reality is that your use does determine how long a tank will last.

Easy way to determine fuel level if your tank does not have a gauge is to heat some water and pour it over the tank. Wait a second or two and feel the tank, starting at the top. It will be warm at the top. When you get to where the liquid level is, it will be cold. You may actually be able to see the liquid level line. Or, you could purchase a stick on gauge so you don't have to feel it although you will still need to use hot water to activate the change to see the level. You could also purchase a gauge that either weighs the tank or reads through the tank wall to determine the level. It depends on how much you would want to spend. The hot water and feel the tank method is pretty cheap.

Another option my be to have a second 20# filled tank for a spare.  We normally camp in the shoulder months April-May and September-November when the temps are often cool and the furnace gets a real workout since we normally boondock, no electric available - as in our national parks.  Last year while in RMNP the night temps often were in the 40's, propane was also used for  HW and fridge. 

The tank was nearly full when we started the trip but on the way home, the tank had to be exchanged, so in that case we got 8 days out of the tank, but that was with pretty heavy usage.  None used during the day, turned furnace off while hiking.  Also, always try to fill at a hardware store or similar where they will FILL your tank.  Avoid going to gas stations where they offer Blue Rhino or similar where you pay a whole lot more to get a tank that is only 75% full.  Two days ago I filled my empty 20# tank that was empty for $ 14.50 at the neighborhood hardware store.

The admonition against swapping tanks seems too harsh to me.  Yes, it's probably a little more expensive than going someplace that will fill them, although the price difference isn't always that great.  Do whatever works for you and suits your convenience.  Just don't eat at restaurants when you're on the road.  Why would you?!  They charge way more than it would cost you to fix a meal in the Pod and you can eat as much as you want when you fix the food!

Yeah, and I never heard of a difference between BBQ tanks and RV tanks either.  I've never seen a label indicating a tank was one or the other.  This is new to me.

TT