I have boondocked extensively in the past....not so much anymore. So I feel marginally qualified to say:
The accuracy of a meter is really not critical for any RV function.
The difference between 3/4 full black tank and almost full is meaningless. When it reaches that stage your next step is finding a place to dump.
the difference between a meter showing 12.5 volts and another one that shows 12.4 is nothing to worry about. Most seasoned RV boondockers can judge the battery condition by the brightness of the lights :)
My AC meter is off by about 3 volts when compared to my Fluke so I just do the math when I look at it. I'd rather spend my time worrying about whether or not the fish are biting...