I have found that even if it’s a cassette toilet there still seems to be a black tank. Usually a 4gal tank. That’s how I get confused cause at first I thought if it had a black tank than it wasn’t a cassette toilet. But I was wrong.
I think you (and/or perhaps the dealers who are notorious for inaccuracy) are confused with the terminology. There are three systems these days. There is the standard flush toilet with a black tank. The tank is not removable... has to be dumped with a macerator or traditional slinky.
Then there is the European style cassette toilet. It has an interior flush toilet, the drains into a good sized cassette that is removed through a door on the side of the van. The cassette is pulled out and emptied into a sewer system. You can see this on the Winnebago Revel:
https://www.winnebago.com/models/product/motorhomes/camper-van/revel
Then there is the more basic camping style portipotty which has been showing up more in the last couple years. Commonly used by small conversion companies.
As to the platform, all three are your basic big Euro style box van whether Promaster, Ford, or Sprinter. My preferred has been the Promaster as it has the widest interior... and I had their smallest (16') converted into a basic camper. Plus it has front wheel drive. I have avoided the Sprinter because it is diesel, many issues with great difficulty finding extremely overpriced service, and it is the most expensive as you have to pay about 10K extra just for having the prestige of that star. (but they will provide you with a free latte while you get that $300 oil change... lol)
I looked at the Midwest Automotive recommended and there is no info on their website about whether there is a black tank... and no photos. It seems to be only the 24' size. Worth giving them a call. One of the models does have a galley. They used to be quite flexible with tweaks, but not sure if they will do this since REV purchased the company.
Roadtrek has a small Sprinter conversion with a black tank...
Also remember that all these vans have low ground clearance... not sure where you are driving to hike.