We also use the Extend-A-Stay, it was already installed so can't help with that...
BUT if you must buy another (= external) tank, consider something larger than a 20-lb tank. We quickly learned that the gas grill size bottle don't last very long at all, especially using propane heat.
We actually use BOTH a 30-lb AND a 40-lb tank with the extend-a-stay. Don't use both at the same time, but switch-over when one goes empty. And we still have the full chassis tank for back-up or emergencies.
A problem with any of these set-ups is that most propane companies won't come to your site (with a truck) to fill-up a less-than 100-lb tank - you must haul them back and forth yourself. The 30-lb is not too bad, but the 40-lb can get heavy on just muscle power. Good luck.