I'd look at the new Costco Visa from Citibank. I'm not a big Citibank fan, but they give 4% cashback up to $7000. That's hard to beat, and you won't beat it with a fuel card account for two vans.
The drawbacks are that a Costco membership is required and the cashback can only be spent at Costco. But if you shop at Costco anyway (and many small businesses do,) it's as good of a deal as you can hope for, IMHO.
Note that even though you can only redeem at Costco, you get the discount for fuel at any gas station that takes Visa. Which is pretty much all of them.
And if you find a small fleet card that does better, I want to know.