Several glow plug issues, seems every 3 months there is a different one that has to be changed, costing about $250 or more every time on the 2018 Chevy Silverado 2500 Diesel.
Suggest to start w/ the standard battery/alternator checkup. Before first start of the day the battery should measure about 12.6 volts. Then immediately after starting the engine, 13.5 - 15.5 volts.
Do you know if they checked for a glow plug control module update?
Tester