Call suburban, tell them that you don't want the tanks any more.
Make sure you are present when they unhook so there is no damage to your connection.
I recommend that you use a pair of tanks, so that there is always a spare on hand. One is disconnected and capped. (Bugs love to nest in the opening...)
Note that for emergency heating you can go through a tank of propane in a hurry. There may be merit in buying your own 100 lb tank, and parking it next to your hookup. You need a friend to horse it there, but it may be there for a LONG time before you need to refill it.
Also: Here in Alberta, you can get an older tank recertified (Pressure test and a new valve) for much less than the new cost. Not worth it for the barbecue size but for 40's and up it may be worth it. Many propane dealers are set up to do this.
A final thought: While $5 a gallon is too much, your project is going to save you only a hundred bucks a year, and the first year of that is gone to get your tanks. Is it worth it?
Keep your tanks painted to stop rust. Either aluminum or white.