Kenworth T800 reverse kills all power

We have a 2012 Kenworth T800. Recently we were operating the truck down some very bumpy washboard roads. When we made it back to our yard as soon as we placed the 18 spd Eaton into reverse the entire truck shut down. All lights in the cab we gone. We traced wires from the battery box back to the starter. We had power to the starter and power to the fuse box under the hood. After tracing wires all over under the truck power came back on in the cab. Started the truck and again placed the truck in reverse and all power gone again.

Any recommendations on where to start? This truck has a 4600 gallon septic tank on the back with a tag axle. Any help would be much appreciated!!

 

18 spd manual

 

That reverse light switch is that on the top drivers side on the Eaton transmission?

 

Pulled the shifter boot off and disconnected the reverse switch. Still no power to the truck. The truck has a tag axle that raises automatically when you put the truck in reverse. Been trying to trace the wiring for the tag axle, but not finding any loose wiring. Any other options to try?

 

Yes I finally got it started with the reverse switch on the transmission disconnected. Placed it into reverse truck remained running. Placed it in neutral reconnected the reverse switch placed it into reverse and right away lights were going off and engine was about to shut down. Placed it into neutral real quick. Then disconnected the reverse switch was able to back it up and drive it up into the shop. As soon as I set the brake I pushed the drivers side window button down and lights all started shutting down and again engine off. I had a multi meter on the ignition hot wire when it was running and it was showing 13-14 volts. After the engine shut down on its own again I checked the ignition voltage and it was down between 7-8. I have a battery charger on the batteries and at the batteries I am getting 13+ volts. Checked the meter reading from the feed to the fuse box under the hood and I am getting nearly the same as at the battery and at the starter. I don't know where to start. I purchased a Power Probe 4 master, but I'm lost on where to begin.

 

Fixed the problem. The fuse box inside the cab had less than 2 volts. Traced the fuse main power feed toward the battery. Found within a large bundle of thick wires the feed wire to the fuse box was badly frayed and hangin on by 2 small strands of wire.