Where can I locate the transmission control unit for a 2012 Chevrolet Express?

First, the starting problem. It sounds like the starter solenoid is going out or has a burn spot and the brushes sometimes come to rest on that. That's why the lights go out when you hit start and you hear no sound. Second, the engine dying while driving could be due to a bad ignition switch. Most mechanics today are not ingenious enough to really inspect anything that remote.

Both have been replaced and tested:-)

P0700 is a very broad code, could be anything from dirty transmission fluid to a complete transmission overhaul. As these transmissions have a lot of shift solenoid failures I recommend you have a transmission shop do some stand alone testing first. Throwing parts at something hoping it will go away is a real budget killer. In my time I think I have only repalced 3 or 4 TCM's that were totally fried. Try an find a shop that's been there for years as they will have the most knowledgeable techs. Stay away from dealerships.

Oh the module is usually located on the firewall right by the brake booster. You will need the numbers off the unit so the parts guy can match calibrations. I don't see it in aftermarket, but I could be missing it.

This sounds like a good old fashioned battery cable connection problem. Most likely the ground cable where it bolts to the frame, engine, or whatever it bolts to is rusty, dirty, or corroded. Check both battery cables at both ends. Make sure everything is clean and tight. Put some Vaseline on the battery terminals where the cables bolt on to prevent future corrosion. With all the electronics on a modern vehicle a bad ground, especially an intermittent one, can cause all kinds of crazy things to happen! Hope that helps! Jim

Yah looks like TCU is this one is inside the transmission, so not attempting to replace that. all grounds and battery cables have been cleaned/checked. New battery, Battery Cables, Starter, BCU, ECU, Ignition switch, ignition cylinder. The no start issue is never present after being off over night. Always show's up after its been in use for over 4 hours. The not start usually does not lass longer then 10 minutes so hard to take it to a shop. At such a loss and the problem will not stay an issue and is intermittent. We have 8 of these express vans and this is the first time we have had an issue like this.

https://youtube.com/shorts/px6aR2QrJJk?feature=shared