I admit to a very strong bias to DIY. I wouldn't have a Class B if it was handed to me! With that said, here's a bunch of my reasons -
-Class B's are way more expensive than buying a cargo or passenger van and doing your own interior. Sometimes 3 and 4 times the money in initial outlay. If I wanted to spend 15 or 20 G's on a vehicle I'd buy new. I don't want that much money tied up in a vehicle.
- they are designed mostly for couples or families for travelling, not long term living. Comfortable front seating but that's about it.
- the manufacturer has attempted to include everything you'd find in a Class A that's much larger - furnace, 3 way fridge, dinette, 3 burner stove, holding tanks (fresh, gray and black), washroom that has sink/shower/toilet. etc. etc. A lot of that stuff I just don't need.
The inclusion of all the above has been accomplished by sacrificing a lot of good storage space that is necessary if you're boondocking and/or living long term in one. Sit in one for a while and do an honest evaluation of where you're going to store all the necessities!
Most still don't come with standing height, they've raised the roof but not put a high top on it.
With a DIY, I have been able to design my own layout to suit what
I need - sleeping area for one, a good sized kitchen because I eat healthy and cook from scratch, seating (swivel passenger seat) that can be used for dining, reclining and for seating while using the laptop and a bathroom area. And storage, lots and lots of storage....
Oh and full standing height!!
I don't need a shower, I can either use campground or solar. I don't need a dinette, there's only me and I really don't like sleeping on a convertible bed, the hassle of making and converting back and forth and the lack of comfort are no no's.
I get to pick my interior décor. I've gone with soft gray vinyl and paint and will use a bright pattern for the drapes and throw pillows.
I'll probably think of more later....