[overlander63]

Start with the battery. Is it charged? Is it good? Will it hold a charge? If the answer is "no", charge and/or replace as needed.
Now that you have a known good charged battery, check the connections to it. Use your test light. Does the light light when you touch it to the positive terminal of the battery, and the negative terminal? (this makes sure it's working). Next, touch the tester to the positive terminal and ground, such as the trailer skin. Does the light light? If yes, move into the trailer, and start checking the fuse panel at the Univolt. If not, go inside and turn on something, such as a light. Now, come back out and touch the negative terminal and skin of the trailer with the test light. If it lights, you have a bad ground.
If you get to this point and everything is good so far, come back and let us know, and I can give you the next steps, such as checking output from the fuse panel.