The missing number is the "loaded voltage" with the furnace and the water pump both running. 12.1v is the voltage after the "bounce back" after the load is removed.
12.1v resting is 50% of capacity. Any capacity. 50% of what? is the question. A battery that has aged out will show good voltage until a load is put on it, when it "collapses".
You could run a little test for that. A voltmeter in the RV would also show that while camping. I like the little ones that stick in a 12v socket, that also have slots for USB charging.
https://www.amazon.com/Palumma-Charger-Adapter-Display-Battery/dp/B0773BYS6P/ref=sr_1_4?crid=2V53IA1NE1JZX&dchild=1&keywords=voltmeter+12v&qid=1606337825&sprefix=voltmeter%2Caps%2C230&sr=8-4
