Plug in power vs. battery power
First, no dumb questions.
Anything that is 12-volt CAN be powered by battery, but will be powered by your converter if you are plugged into 30amp electric.
Some typical 12-volt items on TT include, ceiling lights, slide motors, furnace, water pump, range hood light/fan, bathroom fan, etc.
Some typical items that are only powered by shore power (30amp) would be the electric outlets and air conditioning.
In my opinion, having a battery is important even on a seasonal site, in case of power outages.
2013 Jayco JayFlight 26RKS
2013 Ford F150 Ecoboost FX4
2003 Coleman Bayside / 2011 Jayco Jayfeather X20E -- RIP
2012 Tacoma Sport DCSB / 2013 Tundra SR5 - RIP