Battery charging from Chevy Silverado

[markwick]

I have a 2005 2500 Chevy Silverado with the heavy duty towing package. As we have been doing some dry camping lately, the Monty's batteries don't seem to be charging as we are traveling down the highway. When I am plugged in the battery does not show 'charging' on the inside monitor panel.
Do I have to do something under the hood? eg hook up to an isolator and the alternator - Any ideas,