Yes you do need a 12 volt battery in place. It prevents voltage spikes with a converter or inverter system. A larger battery and inverter system (optional on most higher end RV's and installed on lower end RV's in some cases, but most RV's do not have a inverter) can power 120 volt loads from 12 volt battery, as long as the battery lasts, and the inverter would be used while not hooked up to shore power.
Also if you have a trailer, it is critical to have a 12 volt battery. Should the trailer become disconnected from the tow vehicle, then the emergency brake cable is pulled, and it will activate the brakes from the on board 12 volt battery. No battery, then no emergency brakes!
Are you having problems?
If the battery goes dead when not plugged in for say 3 days, this 'happens' because the CO meter is always on, unless you shut off the battery disconnect. Also the CO meter, propane leak detector and refrigerator combined consume about 35 amp hours daily! This is enough power to discharge a pair of batteries in about 6 days.
So if you have a battery disconnect switch, shut it off anytime you will not have the RV plugged into shore power - at home, unless you are camping of course.
Your RV engine 'should' be charging the coach batteries. However mine stopped charging it, and I had to replace the relay between the coach and engine battery, to make it charge again.
I also have a solar system to fully charge the RV battery. I lived in my RV for many years, and never needed to run the shore power charger, I just left it off, and let the sun charge my battery.
SunElec.com sells solar panels for a great price!
Have fun camping!
Fred.
