So far, it seems that none of the Suburban 1500's can handle it and 2500's will cost me an extra $900 per year in gas
How did you rule out the 1500 Suburban but not the Sequoia? No half ton has the payload you need for 8500-ish pound TT and carrying 8 people. The Sequoia is shorter and has less passenger space than your current Suburban. Stop looking at tow ratings. With a larger family wanting to tow a large RV, there are a few other ratings that will be greatly exceeded well below tow ratings. My 3/4 ton suburban with 2050 pounds of payload and 9400 pounds tow rating would be right at the GCWR and GVWR/payload with 8 people and 8500 pound TT.
A 2010+ GM 1-ton van with 6.0L/6-spd is a great combo. It offers 13-14mpg in mixed driving, 18mpg highway, and plenty of reliable power to carry 8 people, and 1000 pounds of trailer TW. Nissan's NV3500 may work as well, but the engine/trans are a little weaker than GM.