94,3.3 Fuel milage drooooooping off quickly????

I've had my 94 Grand Caravan 3.3L, for a year (Almost) and the Fuel Milage has been a Steady 23Mpg in Southwest Louisiana.
I'm on a long trip around the country now.

The fuel milage started dropping about the time I hit the California coast.
I figured Hiway 101 would make the Angel's huff and puff a little. But the milage kept dropping as I went up the Coast.
Now I'm up in Washington state and the Milage is down to 18 Mpg.

I'd sure like to hear some meaningful comments on this problem.
Do I have something breaking?

The Exhaust seems to be working OK.
It starts and Runs just fine. (it has the Bump shift at some different RPM's though, but it always did)
Tires are pretty new. About Three Thousand miles on them. No wear showing, and a slight pull to the left when the Van is full of people!
No leaks that I can see. No Spots on the white pavment, and no smell of Gasoline.
I need some help from the Experienced crew on this forum.
While I'm waiting, I'll go pick a couple gallons of Luscious Black Berries in the Alley way's here in West Seattle.
Thud.

check the upstream O2 sensor,change the plugs,chech tire air pressure,use K&N filter.just some thoughts. Doug