Probably number one on my list of things to do would be to get a scangauge or ultragauge to monitor the mileage you are getting. That'll help you tweak your driving which is usually where you see your largest fuel economy gains. See the 100+ hypermiling tips link at the top of the page.
Beyond that it sounds like the vast majority of your driving is highway, so I'd look into any aeromod you are comfortable doing. I'd start with a grill block as they're cheap, easy, and effective. By looking at the pictures I've found of the Rodeo, it looks like you should be able to get away with blocking the entire upper grill. Getting a scangauge will allow you to monitor coolant temperatures and maximize the grill block. After that I'd consider a bellypan, rearview mirror deletes, mudflap delete, and then on to the wheel covers etc. When its time for a tire change make sure you get low rolling resistance tires. They normally don't have much if any price premium and can result is a decent boost in mileage. You can check the 65+ mods list link at the top of the page for more ideas. The wiki also has info on mods people have done and generally what kind of results they've gotten MPG wise. Unfortunately we don't have a lot of info on SUVs, so it won't be an apples to apples compairison in most cases.
