That's not likely to be a problem with the fixture (lamp assembly) or the "bulb." I don't know for sure, but 2014 is a bit early for LEDs...but I could be wrong.
Either way, dim says one thing, and "failed" says another.
Failed needs a replacement bulb or, in the case of LEDs, perhaps the entire fixture. It's an all or nothing thing.
But dimmed could be two things:
1) Somewhat unlikely, but the interior of the lamp fixture could be dirty and/or hazed over from moisture intrusion...and possible rust on reflective surfaces (conventional incandescent lamp).
2) More likely, dim comes from a poor ground. Cleaning up and ensuring a solid ground typically deals with "dim."
You might be able to pull your fixture and perform this test.
The way most automotive wiring works is that the chassis (frame) serves as one side of the circuit...ground. The "positive" wires run all the way to the dash/engine compartment in a motor home or to the trailer 7-pin plug. But ground (negative) wires usually just reach down and screw to the frame. Corrosion or a loose screw can lead to dimming of the lights that are not properly connected to ground. That's why one side will work and the other side will be dim or intermittent.
Usually it's easy to spot a ground wire - typically black or white - hanging down and screwed into the frame in an easy to see location. BUT, with "sealed" underbellies, it's entirely possible that you won't be able to simply slide under the rig and find the ground.
If my hunch is correct, any automotive repair shop can deal with this. (If you are good at DIY, you can do it.) A quick demo of how the lights are behaving and they should be able to spot the problem quickly and locate the ground connection easily. The typical fix involves removing the screw, cleaning up the spot on the frame and the connector, and then putting it back together so things are tight and a good connection exists.
One other place to check is where the ground wire connects to the fixture. It's highly unlikely, but the ground wire might be damaged in some way...say, pinched by fiberglass or other hardware so that only a few strands of wire are still intact.
I'm guessing a replacement fixture will set you back a few bucks, and if you rely on the existing ground wire (cut and splice) the problem may not be solved. If you do it right and run the new ground wire all the way to ground, you'll need to open things up just as you would when inspecting for bad ground. You'll have to check the ground connection anyway.
You might save a few bucks and a bunch of time if you check the ground first.
Again, that's a hunch.