As some one responded earlier, depends. In my older rv the weak link is not the genset but th the house inverter charge controller that puts out but a nominal amp charge rate. This can be replaced ( or modified) to charge at a much higher rate, and be "smarter." Its pricey, but worth considering if you are a boondacker. Whatever alternator/relay setup/ gauge wiring is connected to my house batteries from my Y2k V10 motor actually does a pretty good job charging the house batteries while idling. This is important if your gennie fails, or like me in camps where you aren't meant to run a gennie. Yes the RV idling is loud and I'm sure not as efficient as the Onan emerald 3500 , certainly not as efficient as a yamaha or honda suitcase quiet generator. The V-10 in its own way (no taptaptap) is quieter than most quiet generators IMO. As far as charging off the alternator, I have an older diesel stepvan that is basically designed to be a generator It was designed for roadwork. You put it in park, push a button and it revs at a higher idle sending current from its very high output alternator to a major inverter. Enough to power a chopsaw, a bunch of old incandescent lights and a table saw. Thank goodness for LEDs and Lithium battery tools. Even my table saw can now be run cordless. So again it depends. Even when plugged into "shore power" a house stock battery charger may charge less than a trickle, such is the case with mine. If you rev your engine in a pinch you may be able to charge your batt via the alternator pretty quick. As previously posted the type of batteries and how many amp hours you have will factor in. MY fairly low budget utilitarian work around to quicker more efficient charging is "install" of purchased a 40 amp "smart " with 110 amp engine start portable charger ( not a jumper pack, has no battery of its own ) the kind you plug in. I have it "tapped" into the wiring from house battery to an aftermarket high wattage inverter inside the RV cabin, looks like all 0 gauge wiring there. I didn't clip the jumper clamps so I can remove it and use it "dual-purpose" Mine happens to be from West Marine. I've seen a nice one at the Home Despot from dewalt 30 amps and 80 amp "jump" but has an accessory charging option that can be hard wired as well as jumper clamps, usb charge ports and an AC outlet. The black and decker one is the WM's twin. Most of these have a battery recondition mode. I have this hooked up, so that when i have shore power or run my generator I can charge my 200 Ah AGMs quickly and "smart" If in a bind I can use the same unit to jump my rig from shore power or off a generator. I learned the hard way that the DC out from portable generators is super duper nominal. Like for charging RC toy batteries. but when I plug the portable charger into say a 1000 W gennie I can get charged up quickly or jump my rig. So those who answer right away don't do that do this, but rather that like the first respondent stated, depends. Very curious about the devices the earlier poster mentioned that are designed for your engine's alternator to better charge house batts. I'll research that. Neat! bet they are pricey like most marine and RV "stuff" Without being an electrical engineer or electrician one could test there out true charging rates from different sources using an ammeter, It would be awesome if someone posted their own scenario. Perhaps I'll get around to it. Oh yeah, then there is ye old solar. Dont forget to plug your genni's court back into the RV 110 inlet/outlet If you have such a setup. I bought my rig used and had now idea about this. I'm sure in fancier and newer RVs you don't need to do so. Much of my 2000 Four Winds Funmover was built just well enough to get you "into the RV" and your credit card out. Thank goodness the e450/v10 ford chassis and drive train is super solid. Guzzle Guzzle.