If you have a furnace with a fan built in it will drain your battery fast. You would need 2 batteries then for sure if you ever use the furnace without plugging in.
It takes some learning.
I have 2 deep cycle golf cart batteries. But, I like to go dry camping with no hook ups. With my set up I can go 2-3 days running the furnace if,, it's not too cold, and I am careful with usage. I have a 12 volt outlet in the camper near the antennae mounting plate and I plug a voltage meter in there to keep a eye in batter charge level.