Converting a van, cargo trailer, or a gut and rebuild of an RV all require tons of different skills, shop space, proper woodworking and metal working equipment, a thorough knowledge of cabinet making, electrical, plumbing, and metal working, along with a pickiness to build something you will want to use time and time again and not regret every time you set foot in it. In addition, it will require your full undivided attention for a very long time to do the conversion in addition to a bunch of money.
You are comparing apples and oranges here, a van conversion to mimic a Roadtrek, Winnebago Era/Travato/Paseo, or Airstream, or a rolling house with a full bath, permanent bed, generator, etc, that you probably won't have in the van conversion.
The homemade RV will have little resale, the factory built RV will have very good resale in comparison.
Otherwise you end up with something like the Partridge Family.
Charles