You did not say where the marker light was located. I replaced all of the overhead markers (5 amber and 5 red) on my 84 310 with the LED type. They all had one wire coming through the MH skin and the other wire was short and connected to one of the mounting screws as a ground compleating the circuit. The front and rear markers on each corner of the MH are mounted in fiberglass farings and require two wires to make a circuit. Try reconnecting the wires to the fixture and then, If you have a gage connect one lead to the black wire and the other lead to the frame. Check to see if you have any current, if not connect to the white and try again (make sure you have your lights turned on and the bulb is good) if there is current then chances are that the ground wire is defective and needs to be repaired or replaced.
84 310 Limited Motorhome
"It's when we think we know it all, that we have the most to learn"