"I have never used external RAM before"
Keil (and many other compilers)
char XDATA variable_name;

assembly: access with MOVX

"how do I declare the 'Non Volatile External' variable "
As any other XDATA variable, the compiler or assembler does not care.

"how do I retreive the previous (pre-power fail) value ?"
As any other XDATA memory read.

Erik