Dodge Diesel/Cummins Mechanic Advice Needed

I doubt this will be you problem but I have a 2010 ram. Mine started act strange hard to start and when it would the built in trailer brake would so a code. I took it to dealer and they uploaded new soft ware and said good to go. Couple days later same thing. So I start checking trailer wire and scratching my head. I disconnect the Batteries and find out one is got 13 volts other has 7 volts. Charge both and load test and one fails. Replace both and end of problem. Just what happened to me doubt it will fix yours but never know. Bill