Here is a little tool that I made to have access to electricity from my battery
for testing with 12 volts, while inside my trailer.
I made it with a 1141 (burn) bulb and 2 test wires. After removing the glass and wiring, inside the brass part of the bulb, I soldered the test wires and fill it with hot glue. The grey plastic thing is a piece of 3/8 plumbing speedway that I put for extra grip. But, after testing, it is not needed.
To respect the color code of my trailer the white wire is soldered to the brass part, the negative (and ground), and the black one is soldered to the middle contact, the positive.
Now, I can use any lamp inside the trailer and replace the bulb with this to get easy and switched access to 12 volts.
I did not put a fuse, but of course, one could consider that.