2006 GMC Sierra Batteries dead in the Morning

Electrical problem
2006 GMC Sierra V8 Four Wheel Drive Automatic

A plow was put on this truck and in the morning the batteries are dead. When you turn the ignition its just a clicking sound. When you jump it it starts right up and never does it again until the next morning. The owner of the truck seems to think it is the isolator module on the plow set up. A couple of think it maybe some other short or electrical problem. One question is does this vehicle come with its own isolator module? If it does could this be the problem? This vehicle does have dual batteries (2500 hd extended cab model) alot of electronics bells and whistles. Any suggestions or a few ideas to look at.