Any + or - marked on the original switch is irrelevant. It is also irrelevant on the lighted switch unless the light in the switch is a LED. The switch can be wired into the circuit several different ways.
The pump does not and can not "suck all of the power." Simply, you need to connect the light between the 12v
to the pump and ground. Any ground (white wire) is fine for the light. The additional load is trivial.
Since there are 2 colored wires on the switch, I suspect that there is a ground at the pump and the hot wire is switched. That would mean that you need to wire a light between the wire going to the pump from the switch and a ground. If there is no white wire handy, the shell of the trailer is plenty good for ground. When I put LEDs on my outside lights, there was no ground wires anywhere reachable, so I wired the ground to the shell.