Yep, we had a brand new 1976 Taurus when I was a kid and some of the lights had 2 switches. One for 12v (the little tail light bulbs) and one for the 120v (regular 60w bulbs like in your house). There was no 12v without the battery.
When you plugged in the 120v (30 amp plug) I don't think it charged the battery, but it might have (I was only 13, so didn't remember).
The appliances all worked with manual pilot lights on gas and only the fridge on electric when plugged, in.
Not even most 2014 trailers will have inverters that will power the 120v outlets on just the battery..
What you bought is a very vintage RV and will need upgrades to all the systems to bring it up to today's RV standards and that will cost more than just buying a used one with all those newer appliances...
Good luck!
Mitch