I've been plugging the truck in for quite some time, even had it plugged in this morning. I've unplugged everything from that circuit and it trips every time I plug it in.

What would have / could have happened to make it not work like this?

The GFI is a Ground Fault Interrupter (GFI) so you may have a problem with ground circuit in the cord or the plug. The cord is replaceable-- it plugs into the heater at the block. You may want to check it with a meter for a break or shorted wiring. :)

my 04 did that. Everything was fine and then all the sudden the GFI in the garage kept tripping...took it to dealer and heater cord checked out...never found problem...just plugged it in else where in garage...doesn't help you but who knows...

After trying to isolate what was on that circuit, I decided to try a different cord (extension cord). That was the issue. I can't believe I didn't think of trying that sooner. I thought for sure it had to be something with the truck.

GFCI's measure the current imbalance between the neutral and hot leg of the circuit. If the current differential is equal (no ground) then its happy. If there is a 5 ma or higher current imbalance due to a grounded conductor or other path to ground then it trips. Block heaters are a resistance element type device and can develop an element grounds over time. All it takes is 2400 ohms to ground to trip a GFCI but that's not enough of a ground to trip a 15 amp breaker.

another thing is hang the ext cord where water doesnt drip ini t that will trip it everytime!!