it's probably true that the self test cannot guarantee interuption occurs as fast or faster than it should nor that it absolutely positively 100 percent verifies the circuit interuption.
Kinda like you can't absolutely positively guarantee under all possible conditions that a normal breaker is absolutely guaranteed to trip on every overload it should trip on every time.
But, when you hit the test button, if the GFI trips when you hit the button, that's very very very likely fast enough to protect you, and if your paranoid, you can always plug in a circuit tester to see it interupted the line.
Likelyhood that it won't interupt is very very very small.
And if that doesn't satisfy you, just plug in one of the external plug testers with a built in GFI test button and see if it opens the circuit.
And if your worried about old std GFI's in the trailer, what about all the old std ones in your sticks and bricks house?
Some of the early ones didn't interupt on a ground/neutral short, that was a later addition to the requirements.
My experience is that when I've ever had an issue with a GFI it was that it was tripped and couldn't be reset, not the other way around would't trip on a GFI.
