You can do a Google search for engine-driven battery chargers and find many different sizes/capacity. Many are large enough to easily power one high efficiency A/C requiring about 1,000 Watts. I’ve also looked at them but find any generator that doesn’t use the vehicle’s fuel tank to be less than desirable. They are also kind of expensive unless you build your own.
In essence a DC alternator with small engine would be similar to range extender on some electric cars. BMW offers a small unit as an option (not saying it would work for RV because of high voltage). Principle is the similar though.
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