The van shape is basically VW Tranporter type and this was used in Germany by fire brigades. It wouldn't have taken much to produce a roof which could take a ladder (say a school bus with no rear roof lights and a solid roof) and combine with a suitable interior. Bearing in mind the popularity of the fire brigade in Germany, real and Playmobil, I'm sure it would have sold.
Reading your post, I suddenly remembered that there had been a fire brigade version, not by Playmobil themselves but by Antex. Here it is:
https://www.klickypedia.com/sets/13252-ant-firemen-and-truck/Pretty basic but a changed roof would have allowed better ladders, the side windows at the rear could have been blocked in the same way as with the racing support van/3NNN ambulances and job one.