We just traded my 05 Suburban, I never got more then 15 city MPG, not sure why or how the OP gets 18 but whohoo
Until 07 there was a lap belt in the center 3rd row. We now have an 08 but DH drives it and I have an Ody. We needed the towing option too but I hated dealing with getting the kids and out of the Sub and the car seat issues. I have 5, 3 in seats. If you harness 3 you need to either put one in the center 3rd row or lose the ability to flip/slide the 2nd row outboard seat so access to the 3rd row is a PITB. Although TBH I would feel ok (and did) use the center 3rd row TA for a car seat in the drivers side position. In the 08 and other models this is allowed but nothing in writing says it is ok in the 05
The other thing that annoyed me was the center 2nd row kind of dips down and the lap portion of the belt rides up high on passengers, there also is no head rest so although the seat back is high it was never high enough for any of the kids who were tall enough to not need a booster. The 08 is designed stupidly too for child passengers, the center 3rd row has a L/S but no head rest, the seat belt anchors are maybe 10-11 inches apart and it does have a TA but the anchors sit close to 2" from the bight. We have a GN there, one non boostered kid sits on 1 side and another in a Britax Star Rise Comfy. I have an 18m old who rides in a RF MA and in the 05 Suburban (with bench) he had no leg room, the seat is straight up and the way it is designed his legs were squished by 15m. We switched him to an old style base RA and it was better. He rides in that in a Captains Chair in the 08 but that design does give him more leg room too.
I miss driving my Suburban a little but at 5'2" it was just too big for me. For every day driving and getting kids in and out it was tough. I love love love my Ody..sorry that doesn't help you since towing capacity is 3500w/ the tow package.