You should have two 'chassis' or engine cranking batteries (on the lower tray on my Journey). You also have 2 or 3 'coach' or 'house' batteries that run your lights and the other 12volt stuff in your rig. These 'house' batteries crank the generator...if they are dead, you will not be able to start the genset. When the generator is running or if you are plugged into shore power, your 'house' batteries are being charged but not your chassis batteries.
The battery experts do not recommend replacing only one battery in a group. If one of your batteries is shot, probably best to replace all in that group.
'07 Country Coach Allure 470 Siskiyou Summit #31578, Cummins ISL 425; 2014 Ford F150 toad; Air Force One Toad Brake.
Glen Allen, VA; Smith Mountain Lake, VA.