I would abandon that whole concept of attachment and the dual purpose of a generator and a propane tank in the same enclosure.
I'd suggest to weld in two hitch receivers to the vehicle frame. These receivers will offer attachment for a tray or boxes to hold the generator and the propane tank.
Do you plan to run the generator while it is positioned inside the enclosure? Without some severe separation of the generator and it's box from the vehicle, it will shake the whole body skin and be noisy as all get out. I don't think they make enough Sorbothane to isolate the generator's vibration from the vehicle.