Since you were having problems BEFORE you replaced the batteries, you may have a failure on one of the cable ends, running from batts to gen. Remove, clean, check, wiggle, carefully. If they are crimped on, and covered with shrink sleeves, the bad connection may not be readily visible. And, they may check Ok with a meter, but, when you hit them with the demand of a starter draw, the bad, aka, high resistance connection will not allow sufficient current to pass, and start the machine....
Back to a 5'er after 4 yrs fulltime.
Pulling with a '14 2500 ram, 6.4 Hemi.
Montana High Country 31RE