Your big drain is going to be your ceiling lights. Each 12VDC automotive type lamp will draw a good 1A DC CURRENT. Another hugh drain would be the furnace fan which draws around 4-5AMPS when being used.
Your charged up battery should start out with 12.6-7VDC being fully charged.
Here is simplified 30AMP Trailer Electrical Diagram showing what would be on the BATTERY if you are unplugged from shore power. As you can see alot of items are connected to the battery system so that your basics will run from the battery when not connected to shore power.
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Most people will want to have two batteries installed if they are going to do some camping off the power grid giving them at least 220AHs of battery capacity.
We get by just fine with my 255Ahs battery capacity for camping off the power grid and we use just about everything we do at an electric site with the exception of the air conditioner and the high wattage microwave. We have planned it out to use all kinds of 12VDC items and 120VAC items running from an Inverter and have the battery bank only drop to 12.0VDC (approximately 50% charge state) by 8AM the next morning. Then we connect our 30A Shore Power cable directly to our 2KW Honda Generator using at RV30A-15A adapter and can re-charge our batteries back up to their 90% charge state in as little as three hours generator run time. Three hours generator run time usually fits in with all the camp ground generator run restriction times here on the East side of the US. Where we go the generator is never allowed to be used after 8PM.
The main thing we did was to install a good multi-stage converter/charger unit, larger battery cables, additional batteries, and changed out the automotive type bulbs for LED boards.
We have been doing this method of camping off the power grid since late 2008 and are pretty successful about it now.
It does take some planning
Roy Ken