Are you sure you mean pounds and not gallons? The 20 pound tank is usually what you hook-up to your BBQ grill. I've never seen anyone rent these. I think most of the extended stay tanks that I see use are more in the 100 gallon (430 lb) range. The built in tank on my coach holds 35 gallons (28 gallons at 80% max fill) which is about 120 pounds (28 gallons). The following is a little bit on the different sizes. It's the 20 lb (4.2 gallon) size that you exchange at Walgreen's and/or fast food stores.
"20 to 40 Pounds
Smaller tanks in the range of 20 to 40 lbs. are primarily designed for use on barbecue grills and other large yet portable camping/cooking appliances. They can also be hooked up to RVs. Twenty pound tanks hold around 4 gallons, 30-lb. tanks hold 7 and 40-lb. tanks hold 10. These are the most common of the larger size tanks and can last for 8 to 40 hours, depending on the number of burners and BTU output of the grill.
60 to 100 Pounds
These size tanks are noticeable taller than the stocky-shaped 20- to 40-lb. tanks. The tank and gas combined can weigh up to 170 lbs. when filled. Tanks containing 60 lbs. of propane usually equals 15 gallons, and 100-lb. tanks hold 25. Their BTU output can be more than 2 million. Along with cooking and heating, these tanks can be used for tools like acetylene torches."
Read more: List of Propane Tank Sizes | eHow.com List of Propane Tank Sizes | eHow.com