There are lots of smallish point to learn and batteries are one of the more confusing items as not all is as it first seems.
None of us started out knowing the things we know now, so don't consider it odd if you are confused or missing points!
One of those big points on batteries is that voltage readings are less exact than the meter tells us.
Battery voltages can be somewhat like pouring something thick like syrup in a hole in a barrel! If we pour it in and look in the hole too soon, we can get the idea that the barrel is much closer to full than if we wait for the syrup to settle and become stable.
The readings we get with batteries act the same in that we charge the battery by putting power in at the posts and then it takes time for that power to move all through the cells and all the chemicals. If we look too soon after charging or discharging the battery, we get what is often called "surface charge" readings.
I mention this and assume you are a victim as the chemical reaction will only hold near 12.8 volts if the battery has had time to settle down and becomes stable! The 14+ reading is kind of what is built up from recent charging! A really good nice lead acid battery will only hold at 12.8, at best. But to get that amount charged into the battery, we often charge at near 14 to speed the process a bit.
Some points to keep in mind? One indication of what the alternator is doing can be as simple as looking at the chassis battery voltage. As you crank the engine , that voltage will go down , often quite a lot, but as soon as the engine runs and the alternator puts out voltage, it should pop above 12.8 and if you rev the engine, the voltage run up and down. That is because you are seeing the voltage from the alternator getting to the battery and not the actual battery voltage!
If we go back to the syrup story, you are looking at the syrup you are pouring in, not at how much is in the barrel!
The voltage on both coach and chassis batteries should be very near the same as they are connected together when the engine is running. You may get 13+ on both but then when you shut the engine off, the connection is broken and each battery will only show the charge voltage for a bit but begin to go down for as long as 4-6 hours while the settle! You may get 13-14 while running, shut engine down and maybe get 12.9 or more? But if you come back in a few hours the best batteries will only show 12.8!
Big point is how slow batteries operate. It takes a long time, maybe 6 hours to fully charge a really low battery.
To verify a battery is good, we need to put a charger on for at least 6 hours and then wait maybe 4 hours to let it settle. Then if we got it fully charged and it shows up only having 10 volts, we can say it is a bad battery!
Stores can do this faster if we take them a battery we have charged, as they put a load on the battery and watch how it holds up.
Sad to say, but folks do often sell RV when the batteries are needing replaced, so needing new is not uncommon! ARRGH!
Over the years and many Rv buys, I have kind of started figuring in the cost of new each time! And putting a new coach battery in alongside an old will just make the new fail sooner. Not a good idea as it costs more than starting fresh with both coach new. But you can look at chassis and coach as different issues.
Lots of small points to figure and we are all here to try to smooth the road a bit!