Yeah, check the drive train for any drag, what size tires are you running? Might want to drain an refill your diffs with fresh fluid too.
On my last trip (400 round trip) I got 16mpg in my 88 (also the V6 auto) and it is overdue for a tune up, has 31MTs, and I was driving on some hilly windy country roads (lots of speed up, slow down stuff). My engine is also unadultered. I took a trip in early summer, strictly highway and at that point I was actually getting 20mpg. I know when I drive it as a DD carrying about 500lbs+ of tools I got about 16mpg also.