Engine charging vs generator charging depends on the use of the RV. In our situation, we rarely stop overnight at campgrounds and sometimes need to run the generator all night to run the AC. I would prefer not to idle the RV engine for six or seven hours. Just the possibility of CO entering the coach would scare me. I don't know about the new engines with computer controlled fuel injection, but idling an engine for long periods of time could create problems because of an overly rich mixture. Vehicle engines are designed to run a higher RPMs than idle.
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