I assume that if you build out a school bus that you will likely use NEW materials or basically the same materials used by the current RV manufacturers.
I was a contractor and I also have built many furniture grade cabinets and unless you build with SOLID wood, any cabinets or wall materials will use plywood and have glue in their cores. The same goes with whatever you use to level the floor be it plywood or particle board. Basically I am saying that unless you use solid wood for every bit of framing construction and wall covering, you will be no better off than buying new. Perhaps the thing to do is to take her to a really large RV show and let her walk through the models on the premise that you are getting ideas on what to build yourself.
Don't get me wrong, I like building custom things and have converted trucks and vans as well as completely rebuilt an RV but I did it just to get exactly what I wanted. I still had to live with the various materials that are accused of outgassing altho tat is mush reduced since the formaldehyde type glues have been pretty much eliminated.