Here's an old trick for testing a 12V system for a short. First turn off all 12V accessories, then disconnect the negative battery cable. Leave the positive cable connected. Then take a test light, touch one lead to the negative battery post and touch the other lead to the negative battery cable. If it lights up, you have a short.
You can then turn on a 12V accessory such a light, and perform the above test again. The test light should light up.
Note that modern test lights are extremely sensitive and it doesn't take much of a short for this test to work.
You can isolate your short by pulling fuses one by one and watch the test light. If there is no change put the fuse back in and go to the next one. When you pull the fuse of the shorted circuit the light will turn off.
If you have something that can't be turned off, like a clock in a car, the test light may not turn off but should dim enough to be noticeable when you pull the fuse of the shorted circuit.
Same concept could be used to test trailer running, clearance, and brake/turn lights for a short.