Ground Problem - Stumped

Was getting ready for a weekend trip up the coast. When I went to test the electrical between my GMC truck and Forest River Roo trailer (7-pin), the front running lights on the trailer illuminated, but the rear running lights, turn signals, and brake lights did not. So far, using a voltmeter, I checked the power coming from the truck and found that the left turn signal fuse was blown -- I changed that out. At that point, I had a great signal from turn, running and brake lights coming from the truck. When connected however, no change. Next, I located the ground wires coming from the back end of the trailer. They were bundled and bolted to the trailer frame. I separated those and cleaned them up and bolted them separately to the frame -- again, no change. Next, I located the ground coming from the trailer harness (harness is fairly new). This was bundled with two other grounds headed towards the back of the trailer and was grounded to a screw in the box that was bolted to the frame. To test this connection, I separated the wires and grounded them to a provisional ground that I drilled-out and connected to the frame -- very clean. Still no change. One other thing, when the trailer is connected to the truck and I hit the remote lock, the rear lights faintly illuminate momentarily. I am at a loss of what to do next.
Help.