At our summer seasonal site, the campground charges $25 to fill a 20lb tank. The two nearest places to fill a tank charge $17-$23. The farmers market down the street charges $15 for an exchange. We arrive with an empty 20lb tank (hate to drive 800 miles with a full tank). Exchange the tank for $15 and it lasts us the entire season (the last two at least).
I wouldn't even think of leaving the tank unattended. It stays in the car until after I pay.
Folks mentioned bringing the tank in the store. One time at home the place I usually refill at was having a problem and couldn't fill the tank, so I had to go to Lowes to get it exchanged. I waited outside about 5 minutes after I paid waiting for someone to come out, that didn't include the time it took to pull my car around and get the tank out. I finally walked in where I had been watching the manager and someone else chatting. That got me service real quick :) They escorted me right out and promptly exchanged my tank.
-Michael
