Knowledge is key to getting a good estimate.
You need to know the amp draws of each of the items running off the battery - and how many hours of a day they will be on.
Total up the number of amp-hours used in a day. Compare to half your battery's rating in amp-hours. Do not plan on discharging your battery more than 50% (why you used half of its rating) to keep its life expectancy reasonable. This assumes a deep-cycle battery - a starting battery (normal car battery) has a much lower tolerance for being cycled down to 50% charge.
Usually the number of days your battery can support will be less than you want. There are 3 ways to resolve - use less power, install more battery capacity, and/or add solar or other generating capacity.
In our case, the desire was to keep the A-frame at 50-55 degrees with outside temps in the 40s during the day and high 20s at night. The heater fan was/is the driving load since our camper came equipped with LEDs for the interior lights. Our calculation was that a single battery was not going to cut it for 4 days/3 nights. So we added a second battery (size 24 semi-deep-cycle) in parallel when we took delivery.
We have been successful for a single night - all our multi-night campouts in cold temps have been with shore power so far. We may find out more this coming Spring.
I also need to take measurements with a meter to make more calculations. I think the parasitic load (CO/LP detector, thermostat?, power-off on stereo, etc) is higher than I would like.
2008 Hyundai Entourage