I was born well after the true custom van fad ended, but even as a kid I was well aware of it. Our public library's automotive section was outdated to say the least. There was a book all about custom vans from the late 1970s (I think '77 or '78). It was basically a guide to "vanning" with tips, how-tos, and lots of pictures. Being interested in any sort of light truck, I checked the book out. The basic formula was lots of air brushing + "mag" wheels + captain's chairs + carpet + wood. Bonus points for some kind of table, bar, or bed. I found the vans in the book very interesting, and I hate to admit it, but kind of cool. They definitely were cooler than the conversion vans that were in favor when I was little.
Growing up in the late 80s and 90s, I remember conversion vans being everywhere. Not true customs - just aftermarket packages that lacked a lot of the character of their predecessors. They were basically cushy living rooms on wheels. SUVs seemed to rapidly replace these around the mid to late 90s. Now most conversion vans I see are covered in paint splatter and loaded up with tools.