I have driven a diesel pick-up since 2004. I have found the best diesel comes from a local truck stop that does a high volume. I generally use a couple of jerry cans to top off the boat in the spring.
Once the season gets into full swing and the fuel starts turning over in the marina I top off at the fuel dock. It is easier - pumpout & fuel at the same time. Last summer we had an Azimut 70 top off his tanks and grab a cool 1,000 gallons of diesel fuel! That keeps it fresh for sure. I think the cost differential between the off-road fuel sold at marina prices and the over-the-road stuff w/ taxes makes it a wash.