I don't agree the temps are too low...
He stated 80~85F plus a basking area. I read this to mean the ambient temp throughout the enclosure is 80~85F during the day and the baskign spot is warmer... then zt night the temps drop to the mid 70s...
I have an outdoor enclosure set up for my Tegu here in Charlotte NC. He loves morning sun but once the outdoor temp reaches around 90F he heads for a cooler corner or burrows. Then he's active again in the evening.
When the daytime temps stay under 90F he active throughout the day.
Having that experience I keep an ambient temp of 80~85 with a basking area that is warmer than that.
I also use a flourescent bulb for UVB and a seperate incondescent bulb for heat. They overlap so he can bask in both (heat & UVB) or he can bask in either seperately. I find it's common for him to utilize all three zones as well as wonder/explore/relax in other areas of his indoor enclosure.
Sorry for talking so much about my Tegu in your thread... but I like to site the experience that gives me the 'theories' I have with them
All in all... I think it's just the end of the season and he's slowing down. Some Tegus will hibernate while some will simply brumate. Of those that brumate, some will slow down drastically only surfacing once or twice a week for a couple hours at a time, while others will barely slow down compared to summer time activity levels...
It was really awkward for me to get used to the hibernation/burmation periods of my Tegu and I'm still getting used to it. But the best lesson I've learned is to simply watch my tegu and see what it seems like he is trying to do... then provide conditions that help him do it...