I have a van and trailer combo too. Start with smooth wheel covers and and airdam (those US van undersides are terrible).
I put all my 'daily use' gear in my van and put my 'occasional use' gear in the trailer. I leave the trailer at home as much as possible and that's a good 20% gain. If I;m doing really small jobs or quotes I'll take the Prius. On days when I might need one or two things out of the trailer I try to squeeze them into the van. If you can re-organize your gear that would be mod #1, along with asking if you really need ALL that stuff.
I'm also always on the lookout for 2-3in1 tools. My Werner folding ladder takes the place of 3 ladders and a platform (my old ladders are nice for extended use but only come along if I know it's likely to be worth it). A track saw is a lot smaller and lighter than a table saw. Carry enough hardware for a month at a time, not a year or a decade like I often see! If something isn't used at least once a year, I don't carry it.
It does take a bit more planning and I have to think my day through before I leave, but it's let me half the size of the van I needed, which has roughly halved my running costs too.
I get 31MPG city with my trailer.