Low voltage at trailer plug

i wired in a 4 pin trailer plug on my xg its a 1989 4 door 6cyl auto. i had to put in the 4 pin with the converter cause of the amber signals. i noticed when i plugged it in to the triler the lights werent very bright. i measured the voltages at the plug i just wired and the tail lights were 11.5 volts, brakes at 9.8volts and signals like 8 volts. the wiring is good, all soldered and heat shrunk connections, and the ground i verified 0.3 ohms to battery ground. why is the voltages so low there? shouldnt they all be 12 volts or battery voltage? is this normal?

check your ground.

make sure you have a good ground connection.

It sound like you are pulling ground through a rusty trailer ball.

you can run a jumper wire from the tow vehicle to the trailer to see if the voltage goes up and the lights are bright.

Are you checking it on the plug off the converter, or at the trailer lamps? I had similar problems, turned out that the trailer frame/tilt bed were the ground conductors, and the pivot had rusted enough that it wasn't making contact. A short piece of 12ga, connecting the bed and tongue fixed it.

I also had somthing similar the trailer manufacturer didnt hook up the ground on the trailer off the plug.

Many Many Years ago I had a popup that used the ground off the trailer hitch so it was allways hit or miss. It also had a similar problem until I hooked up a dedicated ground line.

If your trailer do not have a dedicated ground line? Put one on even if that's not your problem. (I think it is.) Sooner or later it will become one.
The voltage reading are not that bad.