Just to verify: You mean the chassis battery (used to start the engine), but not the house/coach battery (used to power the RV stuff)? If it's not charging, your alternator or associated circuitry is not working—there shouldn't be any solenoid between the alternator and the chassis battery. One possible problem on some vehicles is the charge idiot light burning out, though I have no idea if the Ford chassis is wired that way.
If you mean the coach battery, typically there is an isolator of some sort, commonly implemented using a solenoid, that is intended to let the alternator charge the hose battery. These solenoids do sometimes fail or need cleaning, among various other problems. Sometimes also they have somewhat sophisticated controls that don't switch them in until after the chassis battery has been decently charged, though that's probably not the most common, particularly in the era you're talking about. There may also be fuses at one or both ends of the wiring or other typical electrical problems (corroded connections, loose wire terminals, etc).
The simplest hookup is with the two terminals of the solenoid going to the two batteries, and the control coming off a line switched by the ignition. Often there's also a pushbutton that powers the solenoid as the "emergency start" feature, possibly connecting the control line to +12V from the house battery (rather than the chassis battery which presumably is drained if you're using the switch).
Sometimes a diode-based isolator is used, which is basically just a high current rectifier; these I gather are not the most reliable, and inherently lead to a voltage drop due to the rectifier's forward voltage (about 0.6V or 0.7V for the usual silicon diodes).
