High brake light?

you can use a converter since the right and left brake actually work in tandem when braking as lit right and left turn signals, or just let your 3rd brake light blink if that side blinker is on. There are 3rd brake lights that blink, so it would be just as beneficial.

You can splice into either the right or left brake light wiring (ground and 12v+) and just run the wire to the third location. You do not have to "balance" between the left and right brake lights by pulling power off each one equally,both get power from the same place anyway.

or, you can just splice into the 12v+ off one brake light and find ground somewhere else if it works better for you; this is just one wire from brake, then one to ground.

Use an LED to keep the amp draw lower.

Get some "fish tape" from hardware store so you can fish the wire through the wall (may or may not even be possible).

Maybe you can remove the rear light lens, and then fish a pair of wires down from your new 3rd light brake hole to the open lens on your trailer. Then splice wires in. This way no wires are exposed inside the camper.

Better is like above guy where he put two lights in, so he has running and turn and brake up high too. It is a little more work, well twice as much work, but no need for an adapter, and you will have a better lit rear.

1,000 ways to go at it, Im sure all will work

I did what 2naeagke did only vertically on my TT.

Not a high brake light.
I added two tail/brake lights to my Jayco.
The wiring came in behind the sofa so it was easy to do.

I'd like one as well. I easily installed one on my cargo trailer, since the rear was pretty open fishing the wire from the brake lights up to a CHMSL was "easy peasy" just ran it behind the drop down rear door spring.

The light I used used each half for a turn signal and both on for brake lights.

On the TT, NOT so easy, haven't figured an easy way to get the brake light wire up there.