To me, it's not worth spending most of your time in the van, for very few concrete hiking. Nature is too unpredictable to hope seeing all the animals you want, whenever you go there. That's just my personal opinion on visiting Khao Yai while in Bangkok.
The tour in itself is okay organized. It's not a unique experience, as it is managed by a tourism company, and you can see other groups there. You can bring your own food as the choice inside the National Park is reduced.