Bob, let me preface this by saying that I'm not an electrician. I have had some limited training with 110V and 220V systems, but that was a long time ago and by no stretch of the imagination did it make me an expert on electrical issues.
In general the GFI circuit will trip if you pull more amps than the circuit breaker allows, or when there's a mismatch in current flow out the hot wire relative to what's coming back on the neutral wire. A mismatch indicates that some of the current is flowing to ground somewhere. The GFI breaker detects this and trips to prevent injury.
Next time in a hardware store, take a look at the extension cords. You'll be hard put to find one longer than 50'. Most are 25' or less. The longer cords are typically using heavier wire - 12 gauge vs 14 gauge. The thicker wire minimizes the voltage drop. If the extension cords being used are 14 gauge, which is quite common, there could be a drop of perhaps 15 volts - about 13%.
Defects in extension cords show up in various ways. One serious problem is when you have a bad connection on the ground pin. If there's some current leakage on an appliance, and my understanding is that often there's slight leakage even on appliances that are working properly, the lack of a good ground circuit causes the leaking current to try to find another path to ground. If you ever feel a tingle or light shock when touching the RV, you've become the ground path and that can be VERY dangerous.
Anyway, if the GFI detects a difference between outgoing and incoming electrical flow, perhaps due to a problem with cords or adapters, it will trip. When checking with an alternate load, if the adapters are bypassed, as they might be, the problem might not show up. If the problem is a marginal connection, I'd guess a low amperage test load might not show the problem.
Hope this helps.
