Apologies in advance for asking questions you've probably seen many times already.  I've seen some answers but all in different topics with slightly different situations.

I'm trying to determine if my 12V batteries need replacement.

I'm a relatively new (2nd camping season) owner of a 2018 RPOD 180.  We tend to camp boondock with it on about 50% of our trips.  Last summer was fine in terms of battery life.  we would get 3ish days without the batteries ever dropping below the 1/3 light on the panel.  I've read some opinions on here about those lights so I just bought a bluetooth voltage monitor, but the little buttons/lights are all I had last summer.

I took the batteries inside and had them on a tender/charger all winter.

This summer we did a boondock trip earlier in the season that was fine.  then a 3 day trip in June with plugin, so no worries.  luckily, I procured a generator just before our most recent trip.  

After that June trip the camper stayed unplugged for a couple weeks, possibly a month.  Then I plugged the camper in to charge the batteries. but at this point, I didn't check battery charge, just plugged in the camper.

On this most recent trip (early Aug) we had a full week with no plug ins.  but between the time plugged in at home and 5 hr drive, I'm confident the batteries were fully charged when we got to the campground.  but after the first day, they were drained down to 1/3 or less.  the water pump was making the lights dim when it went on.  we proceeded to spend the next 4 days plugging the camper in to the generator at least once a day, but watching the batteries drain down to 1/3 or less before we went to sleep.  2 nights they got so low the CO alarm went off and the lights on the radio panel went out.

so my question is... is it true that if the battery's voltage drops below 12.2, it sustains damage and needs testing?  I assume that means like at Autozone.  I'm going to take them in to be tested.  But I wanted to ask for confirmation that they shouldn't fall below a certain VDC.

We have two Everstart 24MS batteries.  I think that means marine starting amps as opposed to deep cycle, which sound like they're not exactly optimal for running camper electronics for as long as possible.  I think they are at least 3 years old but could be older.  But until I left the camper unplugged in July I'm confident they were properly cared for with a tender.

on this recent trip where they were dying, we didn't have any serious load on them other than the water pump.  fridge and water heater were on propane.  no roof fan nor furnace.  no radio and lights were used sparingly.

I was considering a swap to LiFePO4, doing the research on an updated converter any everything... but may just go with a pair of new interstate marine deep cycle lead acid.

Have you checked water levels in the cells? Low water level will cause low capacity.

Good luck sorting this out.