You can bring the batteries to an auto parts store and they might be able to do a load test. But first I’d lift the caps to see where the electrolyte level is. The two house batteries together in the picture (ones that say “Marine”) are wet cells, but the starting battery might not be - don’t try to pry the covers off a sealed battery.
Be careful - the electrolyte is sulfuric acid. Wear gloves and eye protection and have water handy to rinse it off your skin or flush your eyes. There will be three openings under each plastic cap, two caps per battery. After cleaning the top of the battery around the caps, slowly pry the caps off with a screwdriver. Inside you’ll see a little black plastic tab molded into the battery hanging down inside each cell. The electrolyte level should be as high as that tab. If the tops of the lead plates are visible above the electrolyte it might already be too late, but you could still try to save them by adding distilled water and charging them. Use only distilled or de-ionized water. If you use tap water it may ruin the battery due to the minerals in tap water contaminating the battery.
You may not have to replace the batteries if they still meet your power needs, but you’ll have to figure out your power needs.
You’re on the right track - you’ll need to add up all your appliances and multiply by the number of hours they’re used, then compare that to your charging situation.
Some of this post explains how to calculate your needs: