We have owned our RV for around 11 years now, and twice in the past we have been let down on RV trips by the coach batteries not getting charged from the engine alternator when traveling.
The reason was corroded contacts on the solenoid that connects the engine battery to the coach batteries whenever the engine is running.
I now have two voltmeters on the cab dash .... one showing engine battery voltage and one showing coach battery bank voltage. When they don't read the same, something is wrong. i.e. If the interconnect solenoid has corroded/degraded contacts ... meaning that the coach batteries aren't getting charged properly when traveling ... the coach battery voltage reads lower than the engine battery voltage.