All "not for hire" means is that you are "private carrier" hauling only cargo or tools belonging to the carrier (you, or your company). As opposed to a "for-hire carrier" that is paid to haul someone else' cargo. "not for hire" does not mean you are not commercial, just a distinction on how much paperwork you have to have on file at the office, and how much insurance you have to carry. Trust me, I am in the middle of getting legal, usdot, etc.. No fun.
Plus the dot guys know that a truck with huge "rv, private coach, not for hire" stickers plastered all over it are the guys trying to skate anyway. No one is fooled.
Gordy, in your case your truck is truly not commercial, just not in the wise eye of the law until it actually gets to the factory. Did you try playing dumb and just driving right on by the scales with a "who me? commercial?" look on your face, or did you actually stop at the first scale and try do do it legally? Doing something as legitimate as you are, I think I would have carried my contract for the conversion in the glove box and just drove right by the scales. A lot of guys pulling 5th wheel campers with a class 8 truck (on the escapees forum) have had similar concerns and problems when bobtailling. The general consensus over there is just drive by.
Keep us posted on the build.
Dave