The boat you are picking up has electric brakes?
Anyways, set your multi-meter to DC. You should see no voltage when the controller isn't being manually activated, except for a periodic blip on the fancier models which is used to verify when the trailer is connected. If you have the voltage turn all the way up, then you should see 12+ volts at the pin when the manual override paddle is fully engaged. If you test it by pressing the brake pedal, you'll see a much lower voltage. Usually 1.5-4 volts, since proportional units only send voltage in proportion to deceleration.
Did you buy a proportional brake controller? Timer based units will test slightly differently (voltage will ramp up depending on time brake pedal is depressed). And they garbage as far as brake controllers go anyways.
2000 E450 dually V10 wagon