What model year is your Sunline?
In addition to what mainah stated, this may help.
The back wall of newer campers, I'm thinking like early 90's and newer, have a common ground wire for all the rear wall body lights and the tail lights. If that ground connection is corroded and broken, the entire back will DOT lights will not work. You did not mention the tail light or the stop/turn lights as they use the same common ground wire as the 5 red lights at the top of the back wall and the 2 red side lights. If you have stop/turn/tail lights, then that ground wire at the frame is intact but could be bad started at the last junction of a tail light.
Here is a pic of a 2004 T1950 I was restoring with the back wall siding off. I was testing that all the wiring was good before we put the siding back on. You can see how the green body lights and white ground wires daisy chain light to light. You break the first or second light connection and the rest all go out. The wire bundle comes up the left rear side at the bottom., You can see those wires hit the tail lights first then start spreading out all over.
Go under the camper at the back wall. Most times on the left rear (drivers side) there is a bundle of DOT wires outside the black Darco membrane that then goes up into the back wall. There is a white wire in the wall bundle that goes to a frame ground at the back wall. Check that ground connection is good.
The green wire in that bundle coming from the front of the trailer 7 wire cable is for the red marker lights on the back of the camper. The red (left) and brown (Right) wires are stop and turn signals.
If the frame ground at the back wall does not help, then you sort of have to remove one of the red marker/clearance lights and start doing checks. You really do not know if you lost the hot wire or the ground. Power up the camper from the truck 7 wire cable and then probe the green wires with a test light for power. If power checks out, then it points to ground being missing. An Ohm meter check on the grounds white wire at the light to a known good ground will tell if that lights ground is any good.
The wire nuts used often have silicone caulk pumped in them on the DOT lights to help stop corrosion. It may be that one light wire nut is all corroded up that will stop that light and all the rest of it down stream.
Hope this helps and let us know how it goes.
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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