As mentioned, the first thing is to find out the generator rules. Some places let you run it all day, others limit you to two 2-hour periods each day (usually around breakfast and dinner times). That will determine if a generator will be useful or not.
As this is your second reserved trip where weather is a potential issue, I do suggest you do your reasearch on the average weather for an area and time frame BEFORE making reservations.
Campgrounds along I-5 and Hwy 99 are usually hot during the day and night - due to the mountains on either side, they do not get the nightly breezes that drop the temps overnight. For summer camping, I recommend going up into the Sierras or to the coast. The daytime temps will get hot, but nights tend to be very cool.
Stay out of the foothills and the I-5 stretch until October through April. During the off season, they are cool and fairly empty.