ISIS tank levels

I don't know how old your ISIS is, but I did a lot of reverse engineering back in 2004 on a vintage 1995 ISIS , which was DOS based.
Here's an excerpt from my notes:
Volatile data files:
ALARM.LOG
ALEVELS.LOG (needs a chksum file)
CHANDELS.LOG unknown, mostly zero (needs a chksum file)
CKS checksum of something
CV0-3: look up tables for calibration. CV3 is freshwater. CV2 is fuel
LUTS.LST index of CV0-3
*RUN*.DAT: 4 files of historical data
MAINTVAL.LST has mtce timers, status (needs a chksum file) cf MAINTVAL.dalynotes
MNTSTR.LST has text strings for mtce tasks (needs a chksum file)
PRIORITY.LOG unknown, not much in it. (needs a chksum file)
TUNEUP.LOG: run timers: 5 lines of hours, seconds
The format of the tuneup file (for Lazzara boats)
File contents Hours and seconds elapsed on
hhhh ssss Port engine
hhhh ssss Stbd engine
hhhh ssss Port gen
hhhh ssss Stbd gen
hhhh ssss Watermaker
once S-gen hrs and secs was corrupted, plus an extra (sixth) line.
sixth line is optional - it is Isis run time.
checksum file is ascii character representation of 16 bit(or more?, but 16 bits is plenty for
small file) checksum of byte stream in the file. to make a new tuneup (hour meter) file, just
make a new ascii file with 5 lines, each with hrs<spc>0 (cf RGM fix sample) and a new checksum file
with typed in hex number (sum of byte stream of ascii codes)

Even with Windows based systems, the configuration data is still saved in files on the Windows PC. Usually in file names much like what Tom listed above. Safest to always backup your entire ISIS program folder to a memory stick to protect against an eventual drive or PC failure.