I try to keep track of how long I have used a cylinder so I have some sense of when it will need filling. Having a dual water heater and using an electric space heater instead of the furnace means that I dont have to fill the tanks very often. W ehave had our trailer for a year now and I am probably towards the end of the first 30# cylinder.
I do know that Flying J has a minimum that they will fill, I topped one off on the way into FL a couple of years ago and was just below the minimum. Off hand I am going to say that it was around 6 or 7 gallons, either way its much cheaper than filling them locally and although I have never used Disneys service, I would bet its not a bargain. Propane is just another reason that I like Flying J when traveling, my truck actually gets better mileage on Pilots fuel but I think the J is nicer and has more to offer than most of the Pilots.