Here's the Ground Current Test I designed. As you can see it's just a standard automotive 12-volt test light being used "backwards". So instead of grounding the clip and using the probe to look for 12 volts DC. You put the clip on the positive battery post and use the probe to check for ground on your shore power cord. This performs a fairly complicated test very simply. Since it pushes an ampere or two of current through the entire ground path, the light bulb will be dim or not glow at all if you have a loose EGC connection anywhere in your RV's electrical system.
You should start the test on the shore power line itself, then add any adapters or extension cords one at a time, checking each of them for continued ground continuity. Plus you can flex each connection while watching the light to make sure it doesn't blink. I bought my tester with a little alligator clip on the nose for about $10, and it works great for this test.
Note that this test is done with your RV's shore power cord DISCONNECTED from any 120-volt AC power. That's makes this perfectly safe and shock-free, even if you do find an open ground.