We now seem to have more than one set of batteries involved.....
So, if you want a definitive test, here's how to do it:
Pull the batteries from the trailer. Put them on a proper charger and monitor what's going on. Most good shops should have a charger that will give you a readout of the whole charge process. Let it run for at least 24 hours on each battery. Make sure that the charge terminates below 1A after the first 16 hours and does not "surge" after that unless the charger is in equalization mode. You do want to do this with a shop charger and not the converter on the trailer. Snap-on makes a pretty good one ....
Let the batteries sit for 24 to 48 hours and check their voltage with a good multimeter. They should be in the vicinity of 12.7V. If they are below 12.5V and it's roughly 70F, they are trash, replace them.
Now grab a handy 6A load. A two ohm resistor will do fine. You will need to dump 72W into it so it will need to be rated for > 150W. Alternatives are a proper solid state constant current load or a big chunk of copper wire in a bathtub full of oil. Strap the load on and check the voltage on the battery.
Come back every hour and log the voltage. If you need to stop overnight, that's fine. Disconnect the load and start again the next day. Keep logging voltages until you are below 12.0V.
At this point hours x 6A will give you amp hours. Twelve hours is pretty good for a group 24 at this current. You would expect a few more hours on a group 27. If you have a solid state constant current load, the numbers are easy. If you use a resistor, go through the ohm's law stuff to get the proper current for each hour. If you use a big chunk of coper wire in a bathtub full of oil, put a current meter on it ....
Once the first battery is done, hook up the second one and start testing it.
Once you are done with both, they should be within about 10% of each other. If they are significantly different, that's not good. One of them will eventually "kill" the other one.
So you now *know* what the capacity of each of your batteries actually is. Add the two of them together and program that into the monitor.
Next, charge the batteries up again.
This time set them away someplace quiet for a month. Now run the same discharge process on them. Do not recharge them first, just run the discharge the same way you did before. Note the results. If you do not have at least 90% of the capacity you did on the first test, the battery should be replaced.
So by now you do have good data on your batteries. You know the capacity. You know how much they loose in a month. You know they are matched and working well together. Time to put them back in the trailer.
Is there a shortcut that gives full data? Nope.