OK, we have another fillup under the summer conditions. The trailer hitch doesn't seem to have affected fuel economy. You can see the data in my fuel log. But now the car is averaging mid-50s mpg. With the A/C off, it was averaging low-mid 60s. That's a significant difference. You would expect Toyota to install more efficient A/C on a Prius, but whatever.
On a side note, I also replaced the front control arms. That was a nice improvement. The old control arm bushings were worn out. They made annoying squeaky noises. Plus the front end felt sloppy. I'm glad that's fixed. But working in 98 degree heat was NOT fun!
Also, I changed my mind about the pizza pans. I was going to drill holes in the wheels and mount the pans directly to the wheels. But I'll attach them to the trim rings instead. Actually, I think I'll do that before starting the BMW project.