hogfanfs
They must have had a convoy of trucks hauling these projectiles, because I don't see a lot of storage room for them.
If memory serves, there was a heavily modified PzKfz IV that had a box that looked a lot like a Möbilwagen with a crane to hoist the shells out. Even then, that only held line 10 or 12 (just not remembering which).
The whole thing was slung on a speciall railway carrage and moved bodily from place to place. So, like as not, the ammo was in boxcars attached to the train.