What is wrong with the wiring on my trailer?

When I connect the plug on my popup camper to my vehicle the tailights work but nothing else. I have checked the wiring coming out of my vehicle with a tester, checked both plugs, and all the way to the lights on the back of the camper. I have power all along the way. I can get the tailights, left turn signal and right turn signal to work individually, but when everything is hooked up together, it won't work. I'll have tailights, but neither turn signal will work. The plugs are good, and all the bulbs are good. Any ideas?