I love the idea and have considered the same thing for ourselves. Here are the things I would consider:
1) Are you going to keep it attached to your pickup truck? If so, it won't be fun to carry around everywhere. If not, you may have to pay for a separate site at the RV campground for each RV.
2) Looks like you can only get a cassette toilet in the models you linked to. That might not give you much capacity or convenience. I've never emptied a cassette toilet, but it sounds marginally worse than a normal RV toilet.
3) I agree with one of the posters above, A/C can be extremely useful. Especially if you are sleeping in that coffin above the cab. The heat can pile up quickly in that enclosed space.
4) A conversion van might offer several advantages such as stealth capability and the ability to park and use the interior conveniences without going outside in the cold or rain.
You will definitely pick up some additional camping options with this setup but may lose some conveniences as well.