If you have some special need not served by a production model or just like to take on projects...go for it.
The idea that mass produced units are junk is mostly a misunderstanding of design requirements. Our rig is 20yrs old and while it does require some maintenance, nothing crazy. Otherwise, it's perfectly functional and no reason we couldn't keep her going pretty much forever.
Most of the maintenance items by the way are in the mechanical systems (fridge, air/con, plumbing, etc...). You will have all those same systems on a home built rig and they will have similar maintenance requirements.
If you think you can make a one off unit cheaper, better and with decent resale value, you are fooling yourself. Most of the tiny houses I've seen are really meant to sit in one spot being massively overweight and not really designed for much highway use.