I towed with a 2002 Suburban K2500 with an 8.1 ltr engine and 4.10 gears through all the US states. The trailer was 9K loaded.
I have 148,000 miles on it. I still use it to drive to work.
It has been a dependable and reliable car. I've had no engine issues. I replaced the AC at 80K, the U joints at 100K, and the brakes at 120K. Other than that, fluid changes.
I have been thoroughly pleased.
The Suburbans with 8.1 do not have an Allison transmission. They have a 4L85. Only the trucks with 8.1 had Allisons. The 4L85 is a great transmission.
'13 Excel Winslow 34IKE
'12 GMC Sierra 3500HD
Previous RV's: '03 MobileScout Titan 29DBS; '01 Sunnybrook Lite 2708 SLE; '99 Coleman Utah
Previous TV's: '02 Chevy Suburban 2500 8.1 4.10; '99 Chevy Express 1500 5.7