You should connect the red wire on the pump to the positive side of your circuit. The black wire should connect to a white wire. If there is no white wire, connect the black wire of the pump to the skin of the trailer.
I haven't seen where you mention a switch to turn on the pump.
Does the pump make noise when you wire it into the trailer wiring instead of the battery directly?
Are the inlet and discharge ports clearly marked on the pump?
If you disconnect the inlet hose to the pump, does water run out the hose freely?
What happens when you try to run the pump? Do any of the 12 volt
lights go dim?
Do you know how to use a meter to check the voltage?