My employer says we have the option to take a 2nd 30 minute break after working 12 hours (excluding 2 paid 15 minute breaks). They initially said it's a paid break but now claim it's not and I can't find any docs to prove it.

Is it CA State law that it be a paid break?

I'm a driver delivering and setting up durable medical equipment and oxygen.

Actually, the second meal period is required under the conditions noted below:

A second meal period of not less than thirty minutes is required if an employee works more than ten hours per day, except that if the total hours worked is no more than 12 hours, the second meal period may be waived by mutual consent of the employer and employee only if the first meal period was not waived. Labor Code Section 512.
http://www.dir.ca.gov/dlse/FAQ_MealPeriods.htm