Several small points that can be simple but also very confusing!
Your RV has a newer system for doing called BIM (battery isolation manager) . It has replaced what was called a "mode solenoid"on older models but they do pretty much the same but solenoids are an electrically operated "switch" that closes contacts. The Bim is a solid state item which doesn't have the solenoid and has done away with contacts to go bad on solenoids!
Simple and likely work better but less user friendly for repair!
Basic operation?
When camped, the coach and chassis 12VDC systems are seperated. When plugged into power or generator running, there is some form of charger connected to the coach batteries to charge them. Older were converters and only did the change from AC to dc for charge. Then inverters were added to use battery to make DC. Without looking, I would guess you may have a inverter/charger/ automatic power switch!
But they all should do the same on the separating/combining of the two 12VDC systems and batteries.
Plugged in, the coach gets charge but not the chassis, so you can get the chassis battery run down at times. When you need a jump start or boost to get the engine running, you have a switch near the driver which they can push to momentarily connect the batteries together. Bim and solenoid do the same but different equipment.
At the same time there is a connection to a portion of the chassis wiring called igniton hot. When we have the key on and this wire is "hot" the two systems, BIM or solenoid should connect the batteries together. This is when we expect to get a bit of charge back into the coach as we drive.
The place things get confused is when we try to read voltage as we will not get a true story if we miss a few points!
While we have charging from any source and read a battery, we see the voltage high and think it good but that is only the charger sending that voltage and DOES NOT mean the battery is getting or holding that charge!
Charging is a sloooow chemical process that takes hours to go from flat to near fully charged and a fully charged lead acid battery will never hold more than 12.8 volts. Just the way it works, so when you see higher, you are seeing what we call surface charge or the charger itself!
To get a betteridea of charge we have totake the charge off and wait a few hours for the chemicals to settle!.
Several points to help for future. When wanting to charge a weak coach battery, start the RV engine first. The coach battery may be weak and it is where the generator gets power to start, so may fail! But if you start the engine which we are assuming to still be good after camping, then the engine alternator is working to give lots of power to the chassis battery and the system should automatically connect it to the coach battery and that is a lot of power to let the generator start even if you have to crank it for a few tries!
Once the generator is running, it should then be charging the coach batteries and we can shut down the engine and alternator as no longer needed!
Maybe that will clear a few questions of what SHOULD happen, but there may be a problem if the coach batteries ran down while plugged in as that would mean the power is not getting from the charge portion for the inverter/charger or whatever they currently call what you have!
I suggest this for testing.
Start the RV engine. This should give good power to both sets of batteries to show they are connected. Reving the engine may give you 13.5 or higher as the alternator is pushing a charge to both sets!
This can be vital as the next step is to try turning the coach battery disconnect off and back on. This switch sends power to a relay and that relay has to have good coach battery to move and close! A magnet holds this relay in the last position. If we turn the disconnect off and the battery is down, we can't get it turned back on----until we get good coach power to move that relay!
See if all is better when the engine is started first as a way to get good power to it all until the battery gets in better shape.
But if you are plugged in or the generator is running, either should be powering the charge for the coach batteries. If you get the engine running, start the generator and let the genset run as you turn off the engine. Killing the engine will drop that charge coming to the coach batteries and let the generator and charger do that? Look to see there is still a high voltage showing on the coach batteries as they SHOULD be getting charged.
If the voltage still shows not charging like 13+, there is a problem between the charger and batteries to check out.
See if that helps and shout back as this is one of the more frequent things we discuss around here!