You would think a full blown DC electrical system would come with a basic
voltage meter. The trailer didn't. So I ordered a bunch of parts from Powerwerx.com
and put together an electrical panel of stuff. I replaced the cable TV/cigarette
lighter combo. I don't watch TV much, and don't even have a 12V TV.
The lower left hole is a cigarette lighter socket. The lighted power switch turns on the voltmeter and the lower right item which is two 5V USB connectors. That module chews up a bit of power converting 12V to 5V even when it's not in use, so I put it on the switch. Since I can tell the battery voltage with the flick of a switch I can now sleep at night. I'm an electrician, and not knowing these kinds of things keep us up at night.
If you get one of these lighted switches from Powerwerx.com you should also pick up a small tube of J-B Weld. They are built strangely in that there is a tube with threads on the outside that the switch proper presses into. That tube then gets bolted, for lack of a better word, into the hole in the 4-hole panel I bought, or into just a 1-1/8" hole that you drill into your panel.
The tube is way longer than the switch itself. You connect the wires to the switch with push-on crimp connectors. You can't get at the spades on the switch when it is mounted in the tube. You would want to pull the wires through the tube, then connect them to the switch, then press the switch into place. That wouldn't be a big problem, except...
The switch really just presses into that tube and even the slightest pressure from the wiring on the back will pop it right out of the panel. Not very good. If you are careful to cover up the holes in the body of the switch (two where the pivot points for the actuator are and two others that just look like vents) you can mount the switch in the tube and then put a good amount of J-B Weld inside the tube to surround the switch and keep it from coming out. If the wires you are connecting to the switch are very small, then probably SuperGlue would work.
I cut off the switch's tube before trying to glue assemble everything and that made it easier to get the wires connected to the back of the switch. I used masking tape to cover the holes on the back of the switch before cementing the switch in place with the JB Weld.