Quirky camper lights

In the past 4 months I've had the same problem on two separate occasions with the lights on the rear of my Outfitter slide-in camper. Without going into all the details, let's say that I've checked all the bulbs on both truck and camper and they all work and I checked continuity thru the cable connecting truck to camper and that looks good. Contacts on cables and plugs were cleaned, and put back together with a bit of conductive gel. Camper lights come on when the truck lights are turned on, and the turn signals work. Yet when the brake pedal is depressed, all the lights on the camper rear, including the backup lights, come on, but weakly. With a 10 year old truck and camper, I suspect an improper grounding problem somewhere. Would it be a mistake to cut into the cable which connects truck to camper and splice an additional wire into the ground wire, then make a new ground to the truck frame? Thanks. Tom McCloud