class SensorData SD0;
I don't know exactly what this is doing, but I do not think that it is doing what you expect. It is not the correct syntax for declaring an instance of the SensorData class.
A class instance has data and methods.
SD0.readSensor();
Serial.write((byte *)&SD0, sizeof(SD0));
Sending the whole instance, data and methods, hardly seems useful. What is the receiver going to do with the methods?
With no length sent, or start and end markers, how can you expect to know when the complete packet has been received?