I do keep a 100 gallon tank around for the lawnmowers, we mow a lot, and handling small cans got too annoying.  I might understand your filling 300 gallons, as I can often catch a low price, drive to the station with my transfer tank in the pickup, and fill up.  But, delivered to the farm is, for me anyway, the most expensive way to buy gas.  Station price here locally is much cheaper than delivery, but years ago that was not the case.  We used to keep fuel on the farm for cars after we sold our last gasoline tractor, but it was because we were afraid we would come home empty and not have enough to get to a station. 

Maybe you are good enough filling up your tank when prices are low?  Is that the reason?

We sometimes fill the car from the lawnmower gas, if prices are much higher, most of the time we can then fill up the transfer tanks at a lower price when it goes down.  However, we fill the car at a station almost all the time, due to the price being usually less, and let them take a chance for water in the fuel or evaporation.

So, if I might ask, is that why you keep your 300 gallon tank full?