The least costly way is to run a second 20 amp plug for the rear AC. 4,000 watt generator will put out roughly 33 amps, trying to run two AC units would be a gamble and a high risk! Replacing the generator would be very expensive and time consuming. On the road you can run the front AC and dash air to keep cool, at campground run one AC from 30 amp and the other with the 20 amp plug
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