Am I Supposed to See 12V at the Tail Lights?

Artum Snowbird wrote:
OK.. to be clearer perhaps... and this might make more sense. If you are looking at a double filament bulb, one is for the brake/turn and the other is for the running light. There will be two feeds to the bulb, hot for the brake, and a hot for the run. There is no negative in the fixture, only a ground. So, my estimate of what you are measuring might be the voltage drop across the two filaments.

Have you taken the lens cover off yet to see what is going on with the actual filaments?

Explain to me your statement, "There is no negative, only a ground".
Negative AND GROUND is the same thing as it relates to RV 12 volt wiring. To the OP, IF you measure less than 12 volts at the light using the NEGATIVE(ground) wire at the light and the Hot, odds are it is a loose/bad/ ground(negative connection in the trailer. The 7 way bargman at the tow vehicle and on the RV SHOULD have a 14 gauge GROUND(negative) wire connection to complete this from the Tow to the trailer. IF not, then the trailer ball will NOT be adequate to supply Ground(negative) to the trailer. The purpose of the previous posts to use a long Ground(negative) jumper from the RV frame is to verify that the ground(negative) UP to the taillight is not the fault. Also, some Trucks/Tow vehicles REQUIRE a Powered tow adapter to get correct 12 volt plus voltage to the RV light system. This means the Truck requires a 12 volt powered adapter that boosts the trucks lighting voltage to adequately power the RV lighting system. Very few trucks need this type system It is usually needed when using a Car or Smaller SUV. IF all the lights on your RV are bright and only the one taillight is dim, then the Ground(negative) is the fault just for THAT light. Doug