I take it very easy in my '90 Westy automatic.  Never floor it away from
stops, cruise at 55-60 mph, and climb long grades by shifting down into
second and maintaining 3,400 to 3,800 rpms, rather than keeping it floored
at 2,800 to 3,000 rpms for the same road speed.  I too have noticed that I
get better mileage at some altitude, like I-40 across AZ and NM where the
same driving habits will get me 20mpg.

At lower elevations the best I can get is 18+

With 166,000 miles on it I can see mileage dropping a little, but in eleven
years and 161,500 miles that I've owned it my total mileage--every tank of
gas I've put into it--is 17.0.  On-the-road mileage seems close to what it
always has been, but I see a noticeable drop in around-town mileage now.
The most recent tune-up and air and fuel filter changes didn't seem to make
much of a difference.