Mix all ingredients in slow cooker and set on low for 12 hours. Your kitchen will smell amazing!
Process in batches in a food processor and strain through a food mill to get rid of peels and seeds - or use the handy dandy KitchenAid attachment.
If the sauce is too thin, we reduce it (thicken) in a big chili or stock pot on the stovetop for about 30 minutes to an hour. I adjust seasoning at this time too.
Freeze sauce in manageable amounts in zipper bags or plastic containers - or use jars in a bath canner.