higher temps, lower mpg over long trip

[Jack None]

She usually runs around 90 C or so, highway speeds, 85 F or so outside. She usually gets around 25 mpg.

I set out on a long trip (10+ hour days), towing a trailer (1300 lbs gross but getting lighter as I go). First day I was getting around 22 mpg, normal operating temps.

On the third day, I noticed she was down to around 18 mpg, and the temp had creeped up to 100 C. It wasn't particularly hot out, and this was on the flat, going 70 mph. No head wind. Trailer probably about 500 lbs gross.

I notice no other symptoms. Driving fine, exhaust looks normal, I haven't found a fuel leak (checking at idle), no hot hubs or axles. The age-old motor oil leak may have sped up a bit. On the third day I had to add about a quart. I don't usually have to add anything between changes. The water pump may be starting to weep a bit.

I'm not sure if this is relevant, but on the second day, I stopped by a friend's shop, he noticed the air horn seals were dead (the horn connecting air filter housing to turbo), we replaced.

Theories?