Idling to charge is hard on the alternator.
Alternators are typically only ~50% efficient. When the alt is pumping out 500w for your use it is also creating 500w of heat, cooking itself. The easiest way to avoid excess alternator heat is only charge from the alternator while driving. This has two effects:
The most common reason people give for wanting to idle all night is to run the heater or air conditioner. This can have serious effects on the vehicle's engine:
Idling for one hour every day for one year is equivalent to 64,000 miles of engine wear. – source
Idling accelerates engine wear and tear creating a need for more regular maintenance and an increase in the number of oil changes. – source
Because the engine is not operating at its optimal temperature when idling, fuel is only partially combusted, leading to fuel residue buildup on the cylinder walls. This can further damage engine components, including spark plugs and exhaust systems, increasing maintenance costs and shortening the life of the engine. – source
In the long term it is more practical to heat with a propane heater before bed, and a contact heater and appropriately-rated sleeping bag for sleeping. People in very cold climate may want to install a vented heater for overnight heating.