Did you do a lift on the trailer? Specs say it's 10'6"...no where close to 12'.
Basically, you aren't likely to find significant difference in MPG...it takes fuel to make power and it takes power to drag a brick thru the air at high speed.
The best way to improve MPG is to slow down.
Are you set on an SUV? A rear wheel drive 1/2 ton pickup (properly spec'd, not just any) is a nice match for a 4 lb trailer. Plenty of them kicking around and if you get a 4door version, you can comfortably carry 4-6 people and likely will have some payload left for stuff in the truck bed.