Have you looked around on Racingjunk.com? I think you would be better to find a toterhome ready to go. Sell the F450. You're going to spend lots more having your truck built. And I don't think you could build much for living quarters onto that F450. You're going to be overloaded before you put your luggage in.
'03 Freightliner FL112, 295" wheel base, with '03 United Specialties 26' living quarters, single screw, Cat C12 430 h/p 1650 torque, Eaton 10speed , 3.42 rear axle ratio