How to save gas in a 1999 f 350 v 10 dully?

Ok so im planing a family trip to reno nv and where coming from eugene oregon. Its about an 8 hour drive. Where going to take a f 350 v 10 and i know its ok on gas it get about 16 on yhe highway. It has a stright pipe with the converter on it and an k and n fliter. So that helps a little bit and everything has been tuned up on it. I was wondering how can we save gas any chemicals would help i know all the basic stuff like keeping your tires inflatied and that but is there anything that would help thanks!