Larry Clinton At Frankfort, (Central) Indiana
Ralph, don't have any idea of the cost today, but you can have a propane company install a 500 gal tank. If the appearance of a large tank in the yard is an issue to you, these can be buried so only the small "hood" is exposed. I would expect any reputable propane supplier to fill your tank in the time required to empty it, even in bad conditions. They have too do this to customers depending on heat!
Again per what I was seeing when I looked at fuel usage your existing unit is just about as efficient for the power provided as a smaller unit, therefore replacing it with a smaller unit would not give you any benefit that reducing the load on your existing will.