Had the same problem. Cheap trouble shoot is 14 guage wire with gator claps on both ends. Clip one end to the negative side of the chassis battery, with someone at the helm have them try different light functions as you place the other end on the metal edges of the light housings.
If this works just run jumper wires from the light frames to the chassis frame.
The tech who fixed mine said the ground issue is common as MHs have lots of fiber glass and plactic and very little metal in the front cap.
Let us know
Jw
2007 Keystone Challenger 5th wheel, Ford F350 Super Duty 6.7L Diesel, Short Bed, 2 dogs and the cat and rolling down the road full time since May 2014