Honestly I don't think you can lower it but maybe a few inches because of the big tires you're running. A few inches won't do diddle for the mpg's. You already have a sloped front on the TT.
With your F150 Eco you should be in the 10 mpg range. How fast are you towing or is it mainly hills?
Heck I get 8.5-9 mpg towing an 11'6"H 31' 9500 lb TT with a 2500 6.4.
What you might do is measure the distance from the top of your tire to the underside of the TT and see if you can actually lower it.
As for the ride adding shocks would be the 1st option IMO. I've had shocks on my last 3 trailers and it makes a big enough difference to warrant them.
The other option is a canopy for the back of the truck.
Or maybe get some softer leaf springs that will still handle your TT's weight. You could easily be oversprung.
