diesel fuel in Florida

It has nothing to do with fuel. When you arrive in Florida, you get stuck behind senior citizens that are driving 20 mph under the limit. This puts your engine outside it's peak operating efficiency. When you finally get out from behind the senior, you instinctively nail it out of frustration, striking another blow at fuel economy. :)

Probably more water in the fuel in FL as the water tables and condensation factor is high. There also is in most instances a wind out of the South that drop all high profile vehicles on the way in unless you come in on a cold front that's headed South. I would also buy the diesel at a truck stop as the old fuel at some low volume sites probably gets degraded. Then the cold air tends to help in NC as it gets so hot in FL the air is wet coming into the intake and mixing with the Diesel. Moisture and rain plug many filters to the point of saturation cutting off air intake. When it's cold in FL engines seem to have more power.

Down hill from NC to FL, west wind, and the ambient temp was actually warmer in NC.

You're going down hill from NC to sea level in Florida? On the way home you are headed up hill and likely into a N wind? Air temp will have some bearing as well, on tire press, oil temps and engine combustion.

And has been said different fuel blends.
Winter grade diesel here in On drops the mileage.

I buy fuel in NY and I get 8mpg, I buy it down in New Jersey and I get 12mpg, and its cheaper in Jersey.

I lose about 2mpg loaded or not and it happens every time. I come down here maybe 6 times a year camping/fishing and the mpg difference never. Like I said, it isn't a gradual drop off, its from one tank full to the next.

Make sense about altitude, but each state has its own blending standards,(or use the federal) and the blend percentage has a direct effect on the combustion/compression of fuel and torque/power to wheels, otherwise the burn rate of fuel varies with blend of fuel, Flordia has some miminal standards when it comes to engry standards(or they did)
This is based on personsal common sense, which might not be wide spread

The only way you could lose that much mileage is having huge amounts of #1 and or bio in the fuel mix. You're losing close to 20% in mileage if you records are correct. That's huge!

BTW, modern turbo diesels lose very little in power, if any from sea level to high elevations.

Your mileage suffers because air density decreases as altitude increases, But your cylinder volume stays constant. This means you're able to draw less volume (fewer molecules of oxygen) into the cylinders, the effective compression drops and the ECU compensates by reducing the amount of fuel added (to keep the mix proper)therefore using less fuel and less energy at higher elevations.

I thought it was me.