20lb or 30lb Tanks

john in idaho is right about the exchange tanks, in that they are generally a very expensive way to buy propane and you can end up with a nearly expired tank.  There is a bit of a bright side though if you have a nearly expired tank and want one with a longer life.  We had an old tank that was probably the original with our 172.  I needed to refill it, but decided to use the tank exchange system to find a much newer tank.  I convinced the store to let me look through the expiration dates on the tank and found one that was almost new.  The gas was a little more expensive, but the savings realized from not needing to dispose of a tank on the verge of expiration and its replacement with a practically new tank made it a worthwhile effort.  Generally, I look for a Tractor Supply to refill LP tanks.  They are usually pretty competitive on price and most have refill services available.

Depends where you are whether exchange tanks are a good deal or a bad one. We used to live on Hatteras island and the one or two places that refilled tanks charged the same as the tank swap guys did. Also that is a highly corrosive salt environment so by exchanging tanks I could avoid winding up with a rusted out tank. Then too there is the convenience of exchanging vs refilling. You could easily be 5 minutes from a convenient store which exchanges tanks but an hour from anywhere that will refill. You can't exchange 30 lb tanks anywhere I know of.

For all those reasons as well as to keep my lift weights below 50 lbs as recommended by my orthopedic surgeon I wouldnt want 30 lb cylinders.

And yes it is perfectly legal to store a limited number of propane cylinders in your tow vehicle (whether a pickup or enclosed SUV) as long as they are stored upright and strapped in place. I used to carry a second 20 lb cylinder in the back of my Highlander on most trips. Being portable also made it much easier to place it and my generator with propance conversion away from the campsite to limit noise issues.

A 20 lb cylinder holds about 430kbtu of propane. The furnace is rated at 20k btu. So it should run for about 21 hours continuously on a 20 lb cylinder. The actual duty cycle you get will depend on too many factors to guess at. If it runs 25% of the time it would last for about 4 days. If you get a 2nd tank you can just switch over when the first one is empty and go refill or exchange it, no need for guesswork.   

I bought a 30lb tank for running generators at home and it's a lot harder to move around than a 20lb. Not only do you have to move around another 10lbs of fuel but the tank itself is considerably heavier. In retrospect, I would have gone with dual 20s in that application too.