The external AC plug, Every time I plug it in now, it blows a breaker in the garage. I checked the breaker and it's ok. Is the a fuse on the truck ? or what can be causing this problem?

Make SURE that’s the ONLY THING plugged into same circuit to the breaker box - and that the breaker itself is rated for sufficient amperage - try using another circuit / receptacle on a different breaker.. IF problem persists - possible damaged wiring to the engine heater OR the heater itself is defective - shorted internally.