When was the last time the chassis battery was replaced? If original, it has died of old age, just replace it.
When you say “all batteries are connected” are you saying you have 110VAC power to the camper while stored? If you do then you do not have to disconnect the house batteries. You do have to make sure the device that charges the batteries from 110VAC is what they call a “three stage charger”. As mentioned in another response a three stage charger should not hurt the batteries when left on all the time. I check the house battery fluid level every three months or so.