I have a TentBox Go I picked up but haven't installed yet I'll be putting on Elevate Racks. The profile looks like it will be pretty similar to yours. How much has the tent dinged your highway milage?
CCA is calculated at around turning a starter in cold weather. To extend performance in the hybrid, you'll want to look for a high Amp Hour (Ah) battery.
After I recalibrated the meter down around 95%, usually within a MPG. Given the variability in pumps I'd find the kind of precision your seeing suspicious.
Wind is the greatest factor in my experience. Last winter when the middle of the country had a below zero cold snap and a fierce cross wind, my trip from Mid Missouri to OKC that I usually get mid 30s mpg on was down to 28.