Even keyless entry fobs will have an small physical key that you can pop out of the fob and use the physically lock/unlock the doors. With the battery being disconnected the fob won't work for unlocking/locking the doors. I've copied the steps below in order to get your 2021 Chevy Silverado 1500 with keyless entry ready to flat tow:
1. Position the vehicle being towed behind the tow vehicle, facing forward and on a level surface.
2. Securely attach the vehicle being towed to the tow vehicle.
3.... view full answer...