2002 GM Envoy. 190K miles. Excellent condition. 3:42 differential. 4.2 Atlas. PMC trasmission cooler. Trailer is 2000 lbs, with 900 lbs of cargo capacity. So max weight is around 3000 lbs. I know I need to keep it in 3rd. It seems to like 60-65 mph in 3rd. Mostly level driving, occasional grades. Planning on going to Washington State Coastal area from S. California. What kind of gas mileage should I realistically expect? Thanks for any input.