Since you wish to operate in a low power mode for the majority of the time, you will need to use the RTI to periodically wake up the MCU. The timing accuracy you require will determine whether you use the internal clock or external clock for the RTI. This will also dictate the low power mode(s) you can use.
With either clock source, the maximum available period between RTI interrupts will be about 1 second. To achieve longer timing delays you will need to count the number of RTI periods in a separate counter variable. For an unsigned int variable, you could acheive delays of up to about 18 hours.
The MCU would be returned to the low power mode on completion of each RTI processing.