For your supply vehicle, the Germans could have used anything from a horse-drawn cart, a small towed ammuniton trailer, various trucks and RSOs. Something specialized like a SdKfz 250/6
Munitionspanzerwagen Ausf B, a vehicle custom built to haul 60 rounds of 75mm L/48 ammo, were usually attached to Sturmgeschutz units, but I suppose you could have one doing the rounds if you wanted it to.
The horse drawn cart or a truck is more likely for many scenarios, however. These guys weren't always as mechanized as we would like to think.