Do you normally track your mileage? How much worse was this than other lows and from recent mileage?
Could be a bad batch of gas. Could be things like lots of short trips, very low tires, 02 sensor going bad, clogged air filter, or a leak in the exhaust. No towing, roof items, very heavy loads, or something like lots of AC or driving at high speed with all the windows down, right?
We just came up I-5 in the Sienna with a spice cabinet on the roof. It didn't seem to have much air blocking surface area, and I strapped it down pretty far back on the roof. But at 75mph, it apparently did, dropping our gas mileage 20-25%.
Looks like though I have records, I haven't calculated my mpg's in the last year. But in the 8 years before that, my best was 23.9 (don't know how....the next highest was 21.5, and 2 more at 21 or above). But my worst was 13.1, with a few in the under 14 category.
My low ones were likely with lots of 2-3 mile trips around town, with a cold engine and up and down hills. (seeing this since on some of those the tank would last a month or multiple months) Personally I'd check a few of the obvious things (air filter and tire pressure), and then see how it does over the next couple tankfuls. There's always the possibility that it's something more (a cylinder not working, whether due to no spark or something worse), but I wouldn't worry too much about that unless it does end up being consistently low without a reason.
(I'm sure it could also be a lot of other things, but it could be brakes or e-brakes dragging, bearings going out (wheel, etc), or if an automatic not getting into that top gear. You should have pulling in a direction or added noise from some of those)