when you say the battery life shows negative what number is it showing?
that negative sign is more than likely a code
if thats the case then i'm pretty sure this would be an EV100 system

but as was suggested you have either a switch issue in the steering angle or....
your directional contactors are not making completely
OR....
one of your motors is not working because of worn brushes

I would check the contactors first, they are the most likely suspect on those trucks since there are 2 fwd and 2 rev sets

Each motor has its own set of fwd/rev contactors