It can undoubtedly be done, as long as you have lots of time and a fair bit of money, and the necessary skills. Also a big truck to pull it with when you're finished. :) A 40' homebuilt toyhauler is going to be very heavy simply because the common person doesn't have easy access to the aluminum framing to keep the weight down.
And I don't know that it will hold up better since the average builder is also not an engineer and would not know how to figure out all the stresses involved in building for a mobile platform. I think building a toyhauler like you would a stationary house wouldn't necessarily hold up that well to the rigors of moving down a rough, bumpy road/highway.