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When testing DC voltage, connecting the black test lead to a ground is the appropriate practice. Ground serves as a reference point in electrical systems, particularly in vehicles like RVs where the chassis is often connected to the negative side of the power supply. By connecting the black lead to ground, you establish a common reference level, which allows for accurate voltage measurements.

Measuring voltage involves determining the potential difference between two points. If the black lead is connected to a ground, then the red test lead can be used to probe other points in the circuit, yielding the voltage relative to the ground. This is essential for diagnosing electrical issues, ensuring safety, and providing reliable readings in various applications within the RV systems.

Other connection options like the positive terminal or the fuse box do not provide the necessary reference point for a clear understanding of the circuit operation, while connecting to the negative terminal is not standard for measuring voltage as it could lead to confusion and incorrect readings.