Are you familiar with SIP's? They are insulated panels, I won't get into it more than that....which are built like your describing here. You can have a SIP manufacturer make up the thickness you want...they will even put in runs for wiring and plumbing, tanks...whatever. Best of all they use specialized wood, glues and foams.
"If it can't be reduced, reused, repaired, rebuilt, refurbished, refinished, resold, recycled or composted
then it should be restricted, redesigned or removed from production."