Propane also comes from natural gas wells. So prices not solely determined by the price of oil.
When I lived in southeastern UT, the primary source for propane was natural gas, and the area had quite a few natural gas wells, as well as oil, but no refineries for hundreds of miles.
Here in southeastern ID not sure of the source; never asked, since I believe $2.25 a gallon is not a bad price.
And tanks should never be filled to the maximum, propane can and will expand. I know for a 20 lbs tank, they normally only fill it to 4.5 gallons. 4.5 gallons will weigh out at approx. 19 gallons. A lot depends on the temp when filled. Cooler temps equal more propane, hotter temps equal less propane.
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Living somewhere in ID; previously lived in Moab UT; previous to that, don't ask!