Be careful.....many Mfg's run the hot and neg power through the area behind the lights and use those screws as the anchor point for the power to the fixtures. I just replaced all of my marker lights with LED's and the LED's came with pig tails that bypass the screw mounting contacts of the old light. On the rear one's the wires were pulled through a hole on the MH at each light so they were easy to cut and wire directly to the new fixture pig tails using crimp connectors.
However on the fronts all the wiring was hidden behind the fixtures....no holes or exposed wires anyplace....the only way to power the new LED's was to crimp on ring terminals and re-use the screws sticking through the MH wall. I also believe that they are wired in a series like Christmas lights, so power goes from one fixture to another to another. So, if you mess up and lose one of those screws, or spin it too much and break off the wire behind the wall....that fixture won't work, nor will any wired after that one.
One more thing, my new lights were a lower profile, which meant that I had to take my Dremel and cut off about 3/8ths inch off every single screw to get the lens to snap on the base plate. Kind of a pain, but they are all done. I used these off fleabay:
10x Red Amber camper Trailer RV Light 6LED CLEARANCE 4" x 2"Side Marker Assembly | eBay
I can take some pictures if needed...just let me know.........
Btw, I have a box full of the old fixtures and covers....if you want them, PM me...it will just cost you shipping.