2 Coolers? One would be the transmission cooler - but what is the other one? A 4x8 trailer is tiny. Is this just a little utility trailer? How much weight do you intend to tow? If its light and if you are not going far, you may not any additional cooling capability. If you are going to take it on the road, you'll need lights - but you might be able to plug in the wiring yourself from behind the left tail-light.
Transmission coolers do not affect fuel economy.