Paul,
In addition to the circuit breaker that Don mentioned, one more item is in the alternator charge circuit to the coach batteries--that big solenoid mounted on the driver-side firewall. That solenoid is energized anytime the ignition key is on, and it is what connects your alternator output to the coach batteries (through that circuit breaker Don mentioned). If the solenoid fails to energize, or has bad contacts, no alternator current will flow through the circuit to your coach batteries. Either the circuit breaker or the solenoid are prime suspects for failure to charge those batteries (neither have any involvement with the Ford truck battery of course).
Since you have a voltmeter, it should be pretty easy to check both components. With the engine running, you should see alternator system voltage (roughly 14 volts) on BOTH sides of the main solenoid terminals and the circuit breaker terminals. If either component has failed and is "open", you should have the 14 volts on the input side, but only coach battery voltage (12 volts) on the output side.
Mike