Start with loads: furnace blower, lights, water pump, fridge controls and device charging.
READING GUIDEOFF-GRID POWER
RV Battery Basics for Travel Trailer Owners
A practical starting point for understanding battery capacity, charging and why solar should come after load planning.
Battery capacity is useful only in the context of the loads you expect to run and how you will recharge. Before buying more battery or solar, understand your daily energy use, battery chemistry, converter/charger compatibility and available installation space.
What matters in practice
Battery chemistry can change charger settings and cold-weather charging limits.
Solar replaces some consumed energy; it does not remove the need to size storage appropriately.