I'm not sure of the cause of your MPG's but what's up with the trailer?

It seems like a lot of boat to be sitting on a single axle. Especially one that is set so far back. What's the tongue weight? I bet a double axle would make it tow easier.

Here's a pic of my bay boat. It has a t-top on it now. I got 15mpg on my last trip to the coast.

 

towing my new 30' 5vr I 've averaged about 11mpg accross country. I could get about 13 if I slow down to 63mph, but it runs great 70mph adn loves to run it hard at that speed.

 

very good replies. Yes my boat catches alot of air. the t-top is not removable, but it sits pretty parallel with the wind, so i dont think it grabs alot of air. The boat does sit to far ahead of the axel, but i got the aluminum trailer brand new for 1,000 and could not pass it up, and that was 3years ago . The truck handles the boat like a man, just killed the gas mileage. With that said we did run 70mph the whole way.

 

I think the t-top is the big culprit.
Last year, towing my boat, 19' ski boat, 4klbs max incl trlr, I got 14-15mpg consistently at highway speeds, 75-80mph.
I've since added a wakeboard tower w/ racks (no speakers, bimini stays down in the boat, not tied up to the tower when towing). I now get only 13mpg on the last 2 trips, same route.
ULSD may have a little to do w/ it, bit it doesn't seem to affect my empty driving ileage. Just the tower up there makes a 1/2mpg difference I believe.
Going to try leaving the tower folded up on the next run and see if that helps.