I run overnight in a sleeper cab and stop to rest for 3 or 4 hours. Is the AUTOTHERM ERS system able to keep the sleeper warm?

The system is not designed for sleepers for 8 hours of rest.
There is not enough heat energy stored in the system to accomplish this.

However, sleepers with integral bunk heaters can be kept warm for approximately the same time as the cab, which might be sufficient for short rest stops of 3 or 4 hours.

A sleeper kit is available for this purpose and allows the driver to actuate the bunk heater fan.