I *believe* that the Avalanche has two sets of LATCH, center and one outboard.
That truck is such a PITA to install a seat in. We had one for our class and when one of the tests came around I got a 4 year old 41 pound as my scenario. I told the instructor that normally for a 4 year old 41 pounder I'd recommend a harness, but that truck was such a PITA I'd recommend a different vehicle, and if that wasn't possible to put the kid in a booster. Course, we weren't installing with LATCH in there. The tether anchors are WAY down behind the seat, and as I recall you need to pull the bottom of the seat up, attach the tether, put the seat back down, install the seat, and then tighten the tether. With a Boulevard I'd just as soon do it with the seatbelt without the tether. That way you don't have to hassle and you don't need to worry about LATCH.
Ok, I couldn't let this go with no one answering you. Here it is from the LATCH manual.
Center: LATCH w/TA on floor at base of seat.
Outboard: LATCH on passenger side w/TA on floor by base of seat, plus TA on driver side on floor near base of seat.
Notes: To access TAs, fold seat bottom up and push seat back forward. Tether must be hooked prior to installing seat. Long tether strap may be necessary.
TA=tether anchors
Told you, PITA. The Boulevard is completely safe without the tether and with the seatbelt, if you find that easier. That vehicle was just NOT made with carseats in mind.
Wendy