Fuel mileage fluctuation

I no longer have this vehicle (actually a RAM 1500 Laramie), but I have always been puzzled about this one phenomenon when I had it and I am wondering if you can shed some light on it.

This is a truck that I would pull a 12-foot cargo trailer with via a tow hitch, with about 1500 lbs of stuff in it, and the 8-foot truck bed would be packed with stuff too - 8 -10 sawhorses, about 10 breeze blocks and six 6-foot easels. It would also have a toolbox next to the cab with more stuff in it (tools, umbrella stand and two 10 x 10-foot tents).

When I had this truck, it would get an average of 380-400 miles per tank when towing this setup. When traversing along the I-10 through New Mexico, which goes to an elevation of approximately 5,000-6,000 feet, the miles I would get per tank would jump to about 580!

I could never understand why this would happen and it would happen every time I traversed this stretch of highway. And there would be no ill effects that the truck experienced. It just drove like it always did but with this huge increase in efficiency.

I would not have a vehicle service between times when going along this route more than once. So, one can eliminate any variables from this.

Any shedding of light on this would be greatly appreciated, though it does fall outside the remit of a car fixing/problem question. But it has always made me wonder why it would get such great mileage when going through this area.