Solar size replaces energy. Battery size stores energy.
A lot of RV solar mistakes come from mixing up watts, watt-hours and amp-hours. Solar panels are rated in watts, but your camping day is measured in watt-hours. A 400 watt solar setup does not make 400 watts all day; it makes energy over the sunny part of the day. Batteries store that energy so you can use it later.
For "run it indefinitely" loads
Use the 24/7 checkbox in the optional device section. The calculator treats that device as a full-day load and compares it to the daily energy your selected panels should make. If the solar array cannot replace daily usage, the batteries will still run down.
For air conditioners and high-draw appliances
Roof air conditioners, microwaves, induction cooktops, coffee makers and electric heaters can dominate the entire system. A 3,000 watt solar result does not mean that much panel will neatly fit on the roof. It means the daily energy use is huge. In many RVs, running air conditioning from batteries is possible but expensive, heavy, sensitive to weather and usually needs backup charging.
Why panel watts and inverter watts are different
Solar panel watts are charging power spread across the day. Inverter watts are instant 120V output. A system can need a large solar array to recharge batteries all day while only needing a smaller inverter to run the appliances that are on at the same time. Air conditioners are the exception that often need extra inverter surge capacity and a soft start.
For wire gauge and fuse sizing
The wire results use current, one-way distance and voltage drop to suggest a starting copper conductor size. The fuse is there to protect the wire, not the appliance. Final sizing depends on insulation temperature rating, routing, bundling, conduit, terminals, manufacturer instructions and local code.