In any trailer on any tow vehicle (Bicycle or car) you always want the heaviest items as low as possible and at the front.

For instance in a U-Haul - you would put the refrigerator in the part closest to the car, in the center, on the floor.

You may need to add a little weight at the front with a couple of rocks or something if all you are towing is a sleeping bag or lightweight gear. It sounds crazy, but it will keep the trailer from "fishtailing" at higher speeds.