I have an RV AC unit that pulls about 12-14 amps at 120 but my source of 2400 watts run will not start the AC. I think with a capacitor boost in series with the power cord might do the job. Is there such think to just plug in in series with the cord as the generator should run the AC without a problem after it starts?

Have you had the AC checked lately? The motors usually have a capacitor inline. And your generator seems to have enough output to start it if that is the only thing your turning on at the time.

Better check the true output of the generator. 2400 watts is 20 amps at 120V.
What size and how long is the extension cord you are using? If you are using a 16 gauge, 100 foot long extension cord the voltage drop of the cord is probably the problem.

The AC works fine off AC in the garage. Using the generator I plug directly in using the RV cord. I have a unique generator in that I have a hibred Chevy PU with 110 outlets in the bed. Suspect it is highly regulated power but it does not go offline when I flip the switch on the AC, but I only leave the switch on a couple of seconds when I see the AC compressor does not come on. The generator outlets run things like saws, a small air compressor and such without issue.

Have you put a amp gauge on the line to see what it is drawing? Did the AC unit come on right away before? Many can take a little to turn on.