so, again, I am new to this..

Learn as much as you can about how the converter/charger and house battery system and how the 12volt DC powers your appliance controls and alarms, interior lights, fresh water pump, etc. If your RV's house battery cables etc and wiring are still intact, you could try connecting any 12 volt battery to see if the converter/charger is working. If it is working, you could connect a proper deep cycle RV type battery and all your 12 volt items might work. You may need to replace the old converter charger with a modern one that does not tend to overcharge batteries and quickly deplete electrolyte levels. You should also check whether you have a working house battery disconnect switch/latching solenoid device that lets you disconnect the house batteries when rig is parked for some time without 110vac shore power. That disconnects the house battery from parasitic appliance controls and alarms so that house battery won't go dead while rig is parked undriven for a few weeks. Any battery will self- discharge over a month or two depending on it's condition. Note: Most RV's have no provision for charging the starting battery when connected to shore power.
You may have an emergency engine start switch that connects the house battery to the starting battery in parallel. The same emergency switch can help start the RV generator when house battery is not charged enough. See my post about heeding my own advice.