That's not too bad of mileage, but I would have expected a little higher at only 65-70mph. Maybe had a headwind?
My 07 get's 12-13mpg towing a wakeboard boat at 80mph, in the flatlands, with the tower up, it's kind of a sail too. Same mileage pulling my enclosed 2 place sled trailer....very light, maybe 2500-3000# loaded, but 8-6 wide and 8-6 tall is a pretty good parachute.
Still a pleasure to fill up compared to the gassers. Same trailers behind them put me in the 6-8mpg range.

 

The industry standard rated speed limit for ST tires is only 65mph. VERY few trailer tire manufacturers make trailer tires for speeds above 65mph. Those trailer tires that do sometimes have higher ratings are typically heavy duty 16" and up size tires.

It is highly unlikely that a ski boat or a $4,000 enclosed utility trailer would have these types of tires rated for speeds above 65mph.

Just a little friendly reminder.

 

Seems way too low for a 5.9L.....

I hauled a 1988 Airstream Excella triple axle travel trailer (34 ft and around 6900 lbs) from Hebron, OH to Berea, KY. I left out (truck only) with a full tank (only had 17 miles driven out of it) and went 278.3 miles and averaged 17.652 mpg with an average mph of 61.84(Juice w/Attitude on #1). Within 10 miles of my destination, I re-fueled and then hooked up. I drove 288.9 miles and averaged 12.862 mpg with an average mph of 56.64 (much lower mph due to an accident delay) Juice w/Attitude on #2. I also have the H&S DPF Delete, too.

 

What also makes a huge difference are the rounded corners of the trailer. I've done lots of reading and if you can get a trailer with rounded corners, (like the air stream and our star stream rounded front and back) makes all the difference in the world. An example is I towed our trailer on a really windy day, then on the same day, same road, same direction I towed a friends camper (6 feet shorter) with no corner rounding or sloped front of back and it was like dragging a sail behind compared to our trailer.

 

The travel speed was the problem as was the aero quality of the trailer. My trailer was not quite as aero as the Airstream above, but was still 34' and 7500-8000-lbs. 15 mpg at 63 mph, Interstate, over thousands of miles.

I pulled a 6'H x 6'W x 12'L tandem axle U-HAUL trailer recently, with a fair load in that and truck. Made 19-mpg at 59 mph for a trip of 320-miles. Trailer was no taller than truck with bed topper of cab height, both were nose-down in attitude [used a MASTERLOCK adjustable hitch head], trailer was also no wider than truck. No tailwind either.

 

Vehicle speed is for sure a mileage killer. I tow at 100 km/h or 62 mph and I get the best results.

 

Gotta love the 305 triple*nickels !

 

And UHaul trailers have stickers plastered on the fenders saying "45mph max speed". Seems contradictory. I thought U-Haul meant haul @ss!

Seriously though, I didn't know that, but it's a crock. 65mph across KS or NE would be verrry painful. I'll have to look at the tires on my trailers.
Reminds me, why can't one get a heavy duty indestructible trailer tire in 14" rim size? I have D load 14" rim tires on one trailer. I've found one mfr that makes these Kumho 857's.