It went in a hundred small places, which is exactly why you cannot point at one thing. Rough order of what actually weighs the most in a conversion:
Water. A litre is a kilo. A full tank is often the single heaviest item in the van and it is invisible in the build because it was empty while you worked. This is almost always where the surprise is.
Batteries, particularly if they are lead acid. Lithium is a large saving here and it is the one upgrade that pays for itself in payload.
Timber framing and ply lining. Individually light, collectively enormous. Battens, 12mm ply on walls and ceiling, and the furniture carcasses add up faster than anything else, and this is where most people are heavier than they think.
Furniture and worktops. A solid wood or stone worktop can be tens of kilos on its own.
Insulation is usually light. Not your problem.
Everything you put in it afterwards. Tools, clothes, food, gear, books, a bike. People weigh the build and forget the contents, and the contents are often a couple of hundred kilos.
The method from here: weigh it again empty of water and personal kit. That splits the problem into build weight and load weight, and they have completely different fixes.