I have a couple of thoughts on closing up the scoop vent. First, where are the coils on the fridge? Are the on the back or tucked under the bottom? If on the bottom, then I don't think leaving the scoop open does any good. If the coils are on the back, and you have zero clearance between the top and sides of the fridge and the opening it goes into, then leaving the scoop open would probably help with cooling.
Second thought: Do you plan on using the trailer in cold weather? If you do, then leaving the scoop open will create a draft of cooler air being pulled under the fridge, up the back, and out the scoop. Basically, you'll lose some of the heated air from your furnace out of the scoop.
Personally, if I were to install a household fridge in an RV, I would seal off the outside openings and make sure the fridge had enough air clearance around it so it could run properly.
My 2 cents...