I have a 2018 Silverado 5.3 34k miles. I keep losing coolant. The dealership has pressure tested it several times. They have even tested the block itself. But they haven't found any leaks. I just had to replace the thermostat because of the low temp. But there were no signs of leaks or coolant smell anywhere. I even checked the reservoir for cracks or leaks...none! Anyone has any suggestions or advice?