12 V ground connection

A good clean mechanical chassis connection is all that needed to ensure all 12 VDC equipment has the same reference point.

If the equipment ground is poor, voltage levels at the terminal equipment will vary and intermittent/poor/no operation can occur.

Maybe get an electrical person to help you.

Yes any 12v connection can corrode and the higher resistance will decrease it's effectiveness. If you clean the connections use Dielectric Grease when reassembling. You can get a tube at any automotive parts store. This will protect the connection for a long time.

One way to test your connection quality is to connect the load so it is drawing power. Then use your meter set to DC volts, about 20 volt range, measure the voltage across the battery (probably 12 volts), and the voltage across the load. If you get the full 12 V on the load, the wiring is perfect for this load. If the load gets only 6 V it means half the electrical energy is delivered to the load and half lost in the wiring.

Connect the meter across each part, say the negative battery post to the wire connected to it. If it says 1 volt, you have lost 1/12 th of your energy right there. Voltage (electric potential in units of volts) is the energy per unit of charge flowing. So doubling the current will double the voltage and the loss.

You need to clean a good point on the frame or test point one and have a good clean point on the other side of suspect connection.
You first should be on the lowest resistance scale that the meter has. Short the leads of the meter together to get the reading including the leads. Then go between the test points. Reading should be 0 ohms after deducting the lead reading.
Next turn the meter to a low DC volt (12-20 volt) scale. Use test point, reading should be 0 volts between the test points.

I assume you are asking about a 12 Volt, not amp (amperage) ground connection.

Electrical problems are frequently caused by a faulty ground which is generally caused by corrosion. Rust is the most obvious, but it can be lead oxide on a battery post, which is "lead rust" instead of iron rust.

If your instrument indicates a minimum of resistance at a ground, it is testing the ground at a very low amperage. So, yes, a higher amperage test might indicate a complete failure instead of a limited failure that a low amperage test would show.

Bottom line. Just take the connection apart, scrape and clean the surfaces, and reassemble. Ditto with the battery grounds.