Hey that is great news on the possible account! From one Prius owner to another, Waddup!
I do weekly cleans at 7 fast foods and I use heat as well as what @squidskc said.
If you can, look into getting some sort of heater, even renting, as Rowdy advised.
Try the purple Zep straight on damp concrete. Scrub, dwell 10 minutes, rinse. You should see the foam turn brown almost immediately.
The beauty of regular maintenance is that you will see the concrete “respond” to the treatments, in that, years of dirt buildup takes time to remove.
Time, agitiation, soap, heat. Those are your friends in flatwork