Patti wrote:

"I've recently learned (from here) that I should never let the gas go below half full, so I'll have to see if I actually DO get better mileage if I keep the tank above half full." 

What??!! Where did that idea come from?  You don't need or want to get gas every 100 miles.
 
I've driven a Vanagon for over 25 years and 250,000 miles and never had a problem running the tank down to a gallon or so.  There is no need to keep the tank half full in this or any vehicle.  If you have problems below half full, you've got a bad tank.  And, you will be driving around with more weight than necessary (about 42 pounds) half the time with a half full tank, and that will reduce mileage, not increase it.

Stuart
'85 Westy auto, 15 to 18 mpg.