Deepsleep mode Data saving in PIC18
To save more data than 2 bytes accross a deep sleep:

Redesign the application to require only two bytes (16 bits) accross deep sleep.

Use a external battery-backed CMOS RAM.  These take very little power when in standby, but still retaining their contents.  A small coin cell can keep the contents alive for years.

Use a external EEPROM.  It can be completely powered down in deep sleep mode, but this can only be done a finite number of times.  If this device wakes up every second for a few ms, then this is not appropriate.  If it wakes up every hour, this could work.

Don't use deep sleep.  Regular sleep is still very low power consumption.  It stops the clocks, but doesn't power down the RAM completely.