Batteries are made and sold in standard sizes according to BCI (Battery Council Int'l) so all 4d's or any other size are the size they have always been, 20.5x8.13x10".
I'm with the 3 dudes above. Diesel vans have one battery under hood and one on frame rail as the chassis batteries (the vehicle came with them). You are looking for a house/coach battery in addition to these somewhere else. SMB did hide them pretty well on some vans, recessed in fiberglass or plastic bins below the floor line but accessible from the interior.