SO.KFZ LOESCHFZ Register classic car as a normal car? Where and how? Or otherwise?

Moin, we are a small group who would like to buy a transit from 1988 together and use it together. Currently the car is registered as “SO.KFZ LOESCHFZ TRAGKRAFTSPRITZE TSF”. It is also not to be overlooked that it is a fire engine =) Accordingly, the insurance companies do not find the HSN/TSN-combi in their systems, which sometimes leads to direct cancellation and partly to eternally long research. According to vehicle license HSN: 0928, TSN: 156 and 15600000. However, since we do not meet the usual requirements for a classic car insurance (yearly kilometers, garage, only as a second vehicle, etc.) we are probably forced to take out a normal insurance.. Does anyone have anything to do with this? Of course we would like to save costs – so want to keep the H-mark. VG