You have been driving all day, but the kids in the back are happily watching their DVDs, playing with their PSPs, and one is even reading a book. You have checked into the RV park, have paid for the appropriate hookup, and just need to level the rig before you get to join in the peace, rest, and relaxation. This is why you bought that RV in the first place.
What you don't know is that your batteries are already ¼ of the way depleted, the electronics, pending dinner, and associated bedtime routines would burn through all the gas in the tanks for the generator. Rather than rely on yourself for a change, you open a hatch, unload what you need of the 100 foot cord, and in one motion plug back into the world.
The power flows from the power grid, and the most efficient power production facilities in the world, directly into your camper's battery charger as AC power. Should any irregularities arise, lags, sags, spikes, or transients in the power supply, these are felt by the charger alone, as it transforms what ever dirty power is supplied to it, to DC power.
The DC power coming from the charger has a choice, and flows to the point of least resistance. A circuit that is completed will have far less resistance than a battery, so the battery charger will directly power the DVD player and PSP above, bypassing the battery via the wires. Since the battery chargers have more output capacity than the load, they charge the batteries too. Multiple battery chargers can be installed to accommodate the load and need.
AC Power inside the rig is always clean and as perfect as your inverter draws it's power from either the batteries or the DC output from the battery charger. When you install a grid caliber inverter, your power will be as reliable, and stable as any respectable hospital.
1 As you sit and camp, your batteries charge back up to over 90%, safely, without overcharging.
2 Your electronics are powered from the charger, excess output is captured by the batteries
3 Your expensive rig, electronics, breakers, and fuses are protected from the AC system you plugged into by the charger.
4 Usually sized equally to the alternator, no wiring upgrades are required.
5 LED TV's, Computers, medical equipment, and sensitive electronics see clean AC power no matter how poor the source.