Most built-in RV generators don’t have much of a battery charging system. They normally output only enough DC to run their own subsystems with an amp or two left over. In the 1980’s, most built-in generators had a 5 amp nominal charging system. With coaches having converters and “Smart Chargers’ in them, the generator manufacturers have dropped the genset internal charging systems. ONAN’s Quiet Diesel series has an option of the installation of a 10 amp charging system but it must be ordered with the generator set or it costs $$$ to get it installed later. The larger motor coaches with constant speed diesel generators usually have an automotive type alternator as part of the genset. This alternator will charge any battery/s connected to the genset.
The inverter generators normally have charging systems built into them but, if connected to an RV, the ‘Smart’ converter/charger in the coach will charge batteries at a much faster and safer rate. I would only use the inverter chargers for an emergency charging of the truck, car or lawn mower batteries.
The contractor type gensets have the capability of running a household or light commercial level battery charger. They don’t normally have separate charging regulators and DC power windings.
For those of you with dual battery systems. If you’re driving down the road and the main engine alternator goes out. Start the generator (on its batteries) and then turn on the ‘emergency start switch’ (or whatever your coach manufacturer calls it) and continue on your way with the generator and the ‘Smart’ charger feeding the chassis DC systems.