No matter what lights you install, don't depend on the trailer frame for the return ground path. If you are really ambitious, run a ground wire through the frame to each light fixture and connect that to the white ground wire in the trailer harness. If you're not that ambitious, then you can do like I did, and run two grounds from the tongue to each frame rail, then ground the tail light housings with a jumper ground wire to the frame rails. This way corrosion between the tongue and frame rails and between the light housing mount and the frame does not break your return path for the ground. I noticed that my running lights always worked but I had on and off trouble with the tail lights themselves. The running lights were attached to the frame and the fender mounts, but the tail lights were bolted to a piece of galvanized steel that was bolted to the frame with rusty bolt. Probably high resistance there. So I made sure the frame rails were connected to the tongue with ground straps and then grounded the tail light housing to the frame with a short jumper ground wire.
Ultimately the for the lights to work, they have to ground in the vehicle electrical system. With boat trailers 90% of the time light problems are ground problems, and 10% cheap light fixtures that corrode internally. Any wires that are joined with crimp on terminals have to be sealed, you can use liquid electric tape.