Blown GFI

the newer ones will not reset if the line/load is revered or if polarity is wrong. just because the wire is black doesn't mean it's "hot" Meter it.

Indeed, as others have hinted, if your GFCI keeps tripping, you may well have a ground fault and it's doing its job. Replacing the GFCI in that case is not a solution; you need to fix the ground fault.

One way to check for this is to remove the wires from the load terminals of the GFCI (with the power disconnected, of course), and then see if it resets once you reapply power. If it will, then that's pretty much proof positive you have a ground fault downstream.

Check all the outlets on that circuit, had the same problem with gfi tripping turned out a wire had come out of the back of another outlet

Moved from Forum Technical Support

GFCI's have the ability to protect things down line form them. If there are other outlets wired to the output of the GFCI then something else on that circuit may be causing it to trip. Most common things are refridgerators and water heaters but even an outdoor outlet that has moisture in it can cause the trip.

The GFI has it's own breaker. Because they blow so much the breakers get tired. It will "look" like the breaker is ok but they partial break.
Shut all the breakers completely off and then back on again.

AND

Same thing happened to me and this is what I found behind the wall of the GFI. Check all the wires leading to it. Mine actually started on fire.:(

Also if a fault remains in the GFI circuit it will not reset.
What all is on that circuit down stream from the GFI?

GFCI receptacles must have power to reset.
You could have a tripped circuit breaker. Reset all circuit breakers. Turn them all off applying light pressure toward off. Then turn on main circuit breaker, followed by branch breakers.
If rig has inverter, check the breakers on it.