A Victron 700 or 712 BMV with the Peukert factor set to 1.00 and charge efficiency to 100% and once the battery is fully charged, the State of Charge will remain reasonably accurate for about 1 mth, then a full recharge just to reset the 100% SOC to clear any errors that have crept in from small loads and small charge currents the current threshold setting has missed. That one can be tricky to set unless you have well set up cabling that avoids "noise" on the power circuits. Set too high and some loads sneak under the radar, set to fine and noise on the circuit is identified as either a charging current or discharge current.

The quality of the lithium cells will also come into play, so the Peukert factor and charge efficiency might need fine tuning as well if you see unrealistic errors creeping in to the readings.
One important thing I missed, reset the charged voltage to 14v, otherwise the BMV will keep resetting the 100% SOC well before the actual 100% level has been reached.

T1 Terry