I've only got the 20# cylinders, so I'm just exchanging them at the Amerigas places that you see all over the place. Currently paying $20 for an exchange. For that you get a new cylinder and it's at least close to being 'full'... Still cheaper than buying a new cylinder and then having to deal with your old one...
I've got 5 of these things total.. 2 for the trailer and the rest share duties for the BBQ and spares..
So far I've exchanged 2 of them. They were at 11 years old, so figured I'd just get rid of them. the others I have are about 9-10 years old, so probably next year I'll get those exchanged..
Just what works for me..
Good luck!
Mitch