I am glad that it doesn't seem as crazy an idea as I was thinking. It just seems a lot less commonly done, so I was nervous there was something I was missing. But I have seen some great conversions online.
I read a lot here about black water versus composting, and I just can't see a benefit to black water. Plus I'm a tree hugger, so i like the idea of making compost.
I was planning on making a small platform for the back of the van, with a water tank, and a box containing a small generator. I could also see mounting the water tank to the roof. But I will need to verify that won't make the vehicle too tall, and I need to make sure I can properly seal everything..
At this point this is all in the planning stages, and I have a spread sheet with pros and cons of each possible starter vehicle.
Thanks for the input, and I'll take a look at cutaway buses. that would seem to be a good option also.
BTW, this was the kind of step van I was looking at:
2002 Workhorse P42 Step Van FL Cargo 1 Owner 6 5L Diesel Food Truck Mail P30 RV | eBay