Basically in a nutshell the trailer frame is the grounding conductor to the truck which in turn is the grounding conductor to the battery. It has to be complete.
If you go to Home depot, Lowe's, or an electrical supply house get some anti corrosion paste for aluminum wire, it works on corrosion, not just AL wire, and use it everywhere you make a connection. On the trailer you can use a washer under the bolt and the paste under the washer, then cover it with silicone. You should never have another problem. Do this anywhere you had a bolt or screw making a connection.