I have RH Brake light fault and no brake lights, checked bulbs, they seem ok. All other lights working including Reversing lights in same cluster. Checked voltage but nothing when brake depressed.
Sounds a lot. Pretty sure it is just a 12V bulb. I think I checked once when I was doing something with the trailer electrics but don't remember anything that high.
It is, of course, possible to make 18v from a 12v battery using a buck converter but I can see no reason why they would so therefore concur with wammers that your multimeter is forqued!