thats pretty much what I did, I tried to keep it simple,
I weighed my truck very carefully at the truck scales
I calculated every thing on the 16" centers because thats where my bed crossmembers, the vertical posts, and the roof rafter tubes were located. Then I spit the weight of each sheet of flooring, roofing and wall materials to the nearest 16" point.
You do the same with everything, from the weight of the Microwave, generator, cabinets, inverter and everything else I loaded into my conversion. Long things like a/c and water tanks get divided up into 2-3 datum points.
I used a dial hunting scale to weigh some things, others I held while on a floor scale. some materials can be found in the machinist handbook.
The stuff adds up, even the vinyl for the padded walls, weighed like 60 lbs.
Then its all a function of distance from the front axle x weight, just like your airplane balance.
then you subtract the distance from rear axle, just like High School physics, lever arm rotation equations.
I came out really close. to start with...
once I could drive my truck after the stretch, I weighed it again. pretty close...
After my box was built, I weighed it again, I was surprised to see my figures off a bit, so I had to re-figure to get close after factoring changes the box builder made to my design.
Even after all that I'm way heavier on my steering then I would like, but my truck had only a 7000lb front axle so it was close before I started and the stretch just made things that much harder.
Do to the unique design of my rear bedroom adding more behind the rear axle really wouldn't do much on my interior and would just add to my overall length. I wish I could have added 2-3 more feet for leveraging that front axle.
I'll have to re-upload my charts and drawings to another hosting site...
if your are into reading poor grammar and word structure here is the link to my build...
http://www.rvnetwork.com/index.php?showtopic=73051
blizz