The brushes are build into Voltage Regulator.
They last long time on modern alternators and it is the run hr or mileage that counts not the time.
The proper charging should be > 13.5V while < 12.5V would indicate the coach running on battery power.
13V is tricky. That would indicate some charge, but only putting a load would give definite answer.
Easy test is to turn all headlights, blower, rear defroster if you have it and then see how the voltage changes.
For CEL you need to scan the code.
You can buy scanner for $35 those days, or $20 for computer plug, so not much excuse to not own one.