Here's a fairly recent thread that has some links to it for sites that have some of the measurements.
https://vanlivingforum.com/Thread-How-about-having-the-dimension-data-in-an-Importnat-Thread
The thing with stuff like this is that the floor measurements alone don't tell the whole story depending on which van you have.
Walls curve out and then back in. Gas filler hoses get in the way as do wheel wells.
The absolute best way to figure it all out is with a pad of paper and a tape measure. I used little pieces of wood to mark the beginning and end of some spots - like laying a board perpendicular to the wall where the driving compartment trim stopped behind the driver's seat so that I could get an accurate measurement to the back doors.
This will only take you so far though. The frequent use of < and > symbol will be necessary...
Once you've got some rough measurements, know what you absolutely have to have in the way of creature comforts then start laying things out.
Here's a very brief synopsis
of what I went through in deciding on my layout..
For me, having a cabinet for the toilet, a comfortable bed and a fully functional kitchen are absolutely mandatory. Oh, and since I'm all for putting in a swivel base for the passenger seat, a moveable table becomes both the dining room and a place to work on the laptop. I'm lucky I'm short, I can sleep across the width of the van. Twin sized bed goes on the plans first. Height to be determined by not only the height of the wheel wells but also by what I want to store underneath with consideration to how high I can go before I can't sit up in bed. Kitchen will fit from behind the drivers' seat to the front of the bed. Okay, toilet has to go on the passenger side because I'm adverse to putting it in the kitchen...go figure on that one...
. Hmmm, the front of the wheel well is only 6" or so behind the side door opening...18" allowance for toilet cabinet won't fit there...oh wait, who uses both doors to get in...extend floor over part of the step well and build a storage compartment in the step well to give me the room that I need for the toilet...problem solved!! Yay, it will all fit.
The most important part is having a sense of what your must haves, want to haves and don't need.