I believe you misstated. Left and right brake/turn lights are working. Tail lights
and running/marker lights are not working.
You have to have a good ground or the working lights wouldn't work so it cannot be that. All of the running lights and tail lights would not have burned out at the same time so it is either no power from the truck, a bad connection at the plug, or the main tail light feed in the trailer is compromised.
I don't know what your vehicle is but my Tahoe had a separate fuse in a different place under the hood. I think that is true of a lot of GM vehicles and maybe many others too.
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