Marker lights don't work, turn signals do?

You probably have a short somewhere. I had this same problem once but I actually saw when the lights went out. Turns, 4 ways, and brake lights all worked buy no running lights. It was an led marker light that had shorted out. One that was a little flaky most of the diodes were out in it already.

 

agreed you have a short somewhere, the battery jumping down is the short and then it trips the circuit breaker lights go off.

 

If you don't see any obvious damage to one off the marker lights.................the question to ask is do you have Led's or Bulbs in your lights?
If bulbs,take then all out and replace 1 by 1 till the system goes crazy.

With leds........same principal but you need to find a place where you can disconnect them and that might be easier said then done.
If you can disconnect groups off lights.

 

i dont know that i would throw money at it by replacing all the bulbs. pull them out one at at time inspect the wiring and you can unplug the lights one at a time if its a short in the light itself you will find one that no longer trips the breaker. start with the simple stuff first clean the pig tail connection spray it out with some electrical contact cleaner pull out the bulk head plug on the trailer and look at the wiring there. follow your wires the length of the trailer and make sure nothing got pinched or rubbed threw anywhere.

 

I have had similar issues before and found out it was the turn signal switch. I have had that be the case on 2 different trucks.