Total aerodynamic drag for any object is a product of the drag coefficient and the frontal area. You can reduce drag by reducing frontal area or by lowering the drag coefficient. Once long, long ago Car & Driver magazine tested aerodynamic drag for a Pinto (remember those?) by letting it coast down a hill and measuring the distance it went. Then they modified it to reduce the drag coefficient (thinks like putting a fairing over the headlights and taping up hood and door joints), and they got a significant increase in the distance it would coast. So, they kept the frontal area the same but reduced the drag coefficient, and it moved through the air easier.
So, the short answer is that making the front end more aerodynamically smooth will decrease the total drag and the amount of fuel needed to move the trailer through the air. Unfortunately there are plenty of other sources of drag, like awnings, vents, and the cluttered underside of the trailer itself.
