Two things here, one your trailer plug cover on your vehicle should have the correct wiring codes for each of your items, etc marker lights, brakes, brake lights, turn signals ...
Also when you open up the panel inside your vehicle all your fuse placements should be marked on that panel so you know which is which ...
With the trailer disconnect have someone turn on the lights, then turn signals, hit the brakes etc to make sure that the trailer plug is working correctly. Once you know that the trailer plug on the truck is working then you can go to the wiring on the trailer.
With the trailer plug connected to the the truck an the trailer plug sleeve cover off test each wire with a volt ohm tester again as you did when the trailer was not connected and you tested the truck plug. This should tell you if the plug wiring to the trailer is ok and now you know if all is ok there then it has to be in the trailer it's self ...
So now with the lights working (on) you should check the marker and tail lights to see if they all have power to them with the volt ohm meter, once that is done then have someone depress the brakes on your truck and see if they work. Of you have both brake and tail lights then the problem is either the bulbs themselves or the socket the bulbs are in have no ground ...
Ground is very important to trailer lights working, always make sure your grounds a clean and the hitch to your trailer is making ground, lots of times people wonder why they have no lights etc when the problem is that the trailer is not grounded to the truck, thus no ground no lights ....
Hope this helps safe travels