Start with a typical day in your RV, rather than choosing a battery or inverter by size alone. A weekend with regular campsite power can have very different requirements from several days away from a hook-up.
Make a daily appliance list
Write down the appliances you want to run, their power consumption and their likely hours of use. Include the fridge, lights, pumps, chargers and any AC appliances. Check the appliance label or manual; a measured figure is more useful where an appliance cycles on and off.
As a starting point, watts multiplied by hours gives watt-hours. A 40 W appliance used for five hours consumes about 200 Wh. This is an illustration, not a complete system calculation: conversion losses, standby consumption and changing operating conditions also matter.
Plan for charging and peak demand
Consider how many days you want to spend without a reliable recharge. Solar availability, time spent driving and access to shore power all affect the answer. Separately, list which appliances may run together and whether any have a high starting demand.
Share this list with your installer before selecting equipment. They can check battery capacity, inverter capability, charging arrangements and the installation as a whole. Explore the Litherra ecosystem to see how the different parts fit together.