What do you recommend for black tank treatments in this heat?

Happy Camper and Unique are both effective and popular treatments. But nothing will help if you aren’t using enough water. If its starting to smell in the extreme heat use more water each flush, and you could even double up the treatments to see if that helps. BUt, I’d start with adding more water which will in turn lead to more frequent emptying.

I use Camco TST, mainly because they have it at my local Walmart and its cheap and works. But agree with the above, use more water than you usually do and add in whatever treatment you like and you shouldnt have an issue.