What is the idle current draw of a charge indicator?
"State of charge" is not a simple measurement. A DC voltage measurement will give some clues but will show different readings for the same battery when switched between charging, no load, and load. If that is satisfactory then LCD is probably the best technology.



Figure 1. LCD displays are low powered.

A random look at the Lascar 
EM32-1 panel meter which can be configured for 9 to 14 V DC measurement shows a current consumption of 150 &mu;A in that configuration.

$$ P = VI = 12 \times 150\mu = 1.8\ \text{mW} $$

Any LCD voltmeter without a permanent backlight should give similar(ish) results.