21 mpg towing something, were you going 55mph? I have never gotten that good of mileage, I get 18-20 max, going up I-5 from San Fran to LA I can get 20 if I drive under 80 but usually it turns into a speed derby and I get 16 or 17 at 90+. you must be doing something right to get tha mileage.

As for the drop to 13, something is just very wrong. What part of CA do you live in? They do change our formulations for the winter, but I have not noticed any big changes myself, but come to think of it I do get a bit lower mileage these days, but not that bad.