You (OP) are the one who asked about buying your tank from the
supplier, rather than have the propane metered out and billed monthly. I
agree that the thing to do is have the supplier fill it up, then pay
them for 400 gallons. Note: 400 gallons is not a mistake-- propane tanks
are filled to 80% of capacity to allow for expansion, thus a properly
filled 500 gallon tank will hold 400. Also. now (summer) is the time to
do this as the price is usually at it's lowest this time of year. Good
luck Larry