I feel for you! It's hard enough to find the single cause of a single problem. Multiple failures causing a single problem can be terrible to find. I think it's correct to believe that you have two separate problems (running lights, interior lights) that you happen to have noticed at the same time.
Wiggle tests are wonderful. Glad you're doing that.
You're also wise to pay the same attention to Ground as you do to Hot. All too often, we focus only on Hot. If the coach builder has driven ground screws into the coach body/frame, make sure they are neither 1. Loose (as in not driven far enough) 2. Stripped (need larger screw, different hole, replace with Nut and Bolt) 3. Driven but stopped before Screw Head actually binds the Ground Terminal to the Coach (can happen if the metal the screw is driven into, is thick enough to make the installer think a self tapping screw is tight but it hasn't bottomed to the head yet).
You need a Meter, maybe even a Test Light (which sometimes is better).
What's the Make/Model of this Trailer?
