I HAD DRIVEN BACK HOME FROM TAHOE AROUND 6:00PM AND THE VEHICLE WAS PARKED FOR 2 HOURS WHEN WE ATTEMPTED TO OPEN THE SLIDE ON THE 5TH WHEEL. I FOUND THE SLIDE ON THE TRAILER WOULD NOT OPEN DUE TO A LACK OF POWER ON THE 5TH WHEEL. AT THIS TIME THE PIGTAIL WAS STILL CONNECTED TO THE TRUCK. I DISCONNECTED THE PIGTAIL FROM THE TRUCK AND PLUGGED THE TRAILER INTO A 50 AMP PLUG AND THE SLIDE OPENED NORMALLY. I THOUGHT IT WAS STRANGE THAT THE BATTERY HAD DISCHARGED AFTER SUCH A SHORT TIME. AROUND 10:45PM MY GIRLFRIEND SCREAMED THE TRAILER WAS ON FIRE AND WHEN I REACHED THE BACKDOOR THE ENTIRE TRAILER WAS ENGULFED AND THE REAR OF THE TRUCK WAS ALREADY ON FIRE. BOTH THE VEHICLES WERE COMPLETELY DESTROYED ALONG WITH A BARN THAT WAS APPROXIMATELY10 FEET AWAY SUFFERING MAJOR DAMAGE.