There are many ways of doing this. As cost rises, so does benefit.
The ideal system is a battery charger like a Sterling BB2450, if you want to charge 24V off a 24V system. The cable to the charger comes directly off the + pole of of the "high" battery in the engine compartment, or the alternator stud. It is programmed to stay OFF unless the voltage rises to 27.2V, which is not possible unless the engine is running and alternator working correctly. There is a 2 min delay to apply some unimpeded charge to the starting batteries.
The charger "soups up" the voltage a bit, like to 14.0-14.5V depending on what type of "leisure battery" you have (conventional Pb-acid, AGM, etc.). That allows faster charging. When the engine goes off, the charger shuts down.
The system comes with temperature sensors for the alternator and leisure batteries, for protection.
I think the 2424 does a maximum of 25A; my two 1212s each do ~45A; I have 4 X 4D AGM batteries, totalling 840 A-hr.
Most other systems are basically "dumb" or "smart" relays, but none bump up the voltage to allow faster recovery of the leisure batteries to my knowledge.
As far as what to do in the boonies if the system fails: just hook the charging cables from alt-charger and charger-leisure battery together with a bolt after engine starts, disconnect and carefully insulate from grounding when stopped. There's usually a fix when complicated stuff breaks down, which is unlikely to begin with.
Unimog U500 w/Unicat