In a small bowl, combine all the spices: garlic salt, seasoning salt, kosher salt, pepper, sugar, garlic powder, and parsley.
Trim the fat cap from the roast.
Drizzle the worchestershire sauce over both sides of the beef. Massage in. Repeat with Olive oil. Massaging into both sides of the Tri-tip.
Sprinkle the spice mixture all over the roast, and rub into the meat with your hands. Place meat in a large Ziploc bag and seal the bag. Place in the refrigerator and marinate, turning a couple of times, for about 8 hours.
Remove meat from the refrigerator 30 minutes before grilling. Preheat your grill to high heat. Preheat for 15-20 minutes to heat it up.
Place the meat on a plate and discard the marinade. Grease the grill.
Sear the roast over high heat for about 2-3 minutes, until nicely browned. Flip over, and sear the other side for 2-3 minutes. Keep the lid shut as much as possible.
If there are any flames, extinguish with water bottle. Turn off the heat on the side of the grill that your meat is on. This will allow you to cook over indirect heat. Lower the other side of the grill to low heat.
Grill without opening the lid for about 20-25 minutes, or about 10-15 minutes per pound, depending on how rare you want it. Rare is 135 degrees F. Medium rare is 145 degrees F. Use a meat thermometer inserted in the center of the roast to check the temperature.
Remove the roast from the grill. Cover with foil and let it rest for 10-15 minutes.
Slice against the grain and serve.