Determine first if this is a sirloin or a sirloin tip roast? The sirloin - from the loin section will cook quicker- because of it's shape than the rounder sirloin tip. I am betting it is the sirloin tip (my favorite) which sells for about 20% of the price of a loin.
In either case, I don't use a rotisserie and, instead, put the roast on a rack over a shallow pan. Then place about 1/4" water in the pan to keep the first juices from burning.
Trim, wipe dry and rub a 10 to 12 pound sirloin roast well with olive oil then follow with a good rub down of:
1 Tbs. salt
2 tsp. garlic
2 tsp. onion powder
1 tsp. ground bay
1 tsp. ground thyme
1 tsp. ground mustard
1 tsp. fresh ground black pepper
Preheat the grill to 350 and bring the roast to room temperature (65-70 degrees) at the center.
Cook the roast with indirect heat by placing it in the grill/cooker on the side away from the heat. How long depends on the temperature that you can maintain in your grill/cooker. I would check with a thermometer probe at about 1 1/2 hours. When the center reaches around 135, take it off and let it sit for about 15 minutes for the juices to set.
Deglaze the pan and scrape all the drippings into a sauce pan and reduce it while adjusting taste with the same seasonings. This juice is delicious.
Slice thinly across the grain.