You probably do have LED's. If you aren't positive which you have, it's kind of hard to describe the difference in appearance. The light produced by LED's is brighter, more harsh, and they go on and off instantaneously because they're actually an electronic device as opposed to a wire filament that has to heat up and cool off. If you've ever been behind a newer model car at night at a stop light and thought their tail lights were especially bright, even too bright, they were probably LED's. Incandescent lights look more natural, aren't as bright, and take a fraction of a second to go on and off.
BadgerRPh wrote:
The logic used by these folks was that if you were getting turn signal lights but no brake lights on a side it had to be an issue with the vehicle as the trailer uses a single wire/bulb/connection for both of these functions.
That's true, and a good point.
You might put your volt meter on pin 6 on your trailer connector (truck side) and see what you get there when you apply the brakes (alluding to Paul's suggestion).
But the more I sit here and think about this, the more I also suspect the flasher, even though I've never in 33 years of driving heard of one failing.
Faulty grounds are the source of 99% of trailer light problems, but I don't think it's the case this time. That light housing has a single ground connection to the frame of the trailer, and all the other functions in that housing are working.