Fiat never made a single motor home, all they did was supply the basic van for conversion to conversion companies of which there are a lot.
So it will depend who built your van as to where things were placed, Even vans from the same year and the same companies can have things in different places.
Usually though with all conversions the split charging relay will be in close proximity to the leisure battery and usually were other electrics such as a fuse box mains hook up or other electrics are kept in the van. so look around the leisure battery to begin with, find the biggest red wire off the leisure battery and follow it till you get to the split charge relay.