Dunno, if the objects are solids (closed volumes) and you are able to BooleanUnion everything - which you should if things are built and fit correctly - you will end up with one outer volume and (perhaps) any number of inner disjoint volumes. I say “perhaps” because in your example, when I BU the parts, the inner volume just disappears “automagically”. IF there are inner volumes that do get left, just unselect all, select the outer volume and invert/delete. Good to go.
–Mitch