I ran a Bedford CF camper years ago that developed the same symptoms (ok it was an old, old carb only engine) turned out it had a relay on the starter that prevented all the available battery power from going to the starter and not leaving enough for a strong spark. I would guess that a starter with slightly decaying windings could well draw more than its share of available current? . The Cf would bump start in a few short feet, but even with the battery fully charged, would not fire off the starter.   If your vehicle will bump, but not start on the key I guess its a good starting point to change the starter, not something I'd have thought of without your post, even withmy previous problem!