They are basically a "park model" RV. The park models typically omit the holding tanks and get plumbed directly to sewer. Of course since they are custom made, it's certainly possible to add a holding tank (in which case they would qualify as an RV).
Reality is beyond people who just like projects, they are pretty silly when you can get a new RV that has all the mod/cons for half the price and it can go down the road at 65mph on a regular basis.