While your feedback is appreciated, I am puzzled as to what you a describing as the cause of the 80A fuse blowing.
I can see that none of the connectors on the RH of the pictured connector block are fully inserted, and that the lower pair being loose, could possibly make a short circuit. If such a short circuit was blowing fuse F71 (80A), then it suggests that the motorhome habitation 12V system has been incorrectly connected to the vehicle 12V system.
Standard practice is to connect at the starter battery, with a fuse as close as possible to the point of connection. On the x244, and I think the x250, Fiat designate fuse F72 as a converters fuse with a 70A midi fuse installed. This option is frequently ignored. On my PVC I have reduced the fuse rating to 50A, and used F72 as the connection point.
Absorption fridges typically draw about 15A when selected to operate on 12V, and unless a supply is available from the vehicle alternator the supplying battery will be discharged. Hence the supply to the fridge is usually controlled by the D+ signal (alternator field connection). The actual power supply for the fridge element is frequently taken from the habitation 12V system, but this means that there will be an additional voltage drop across the split charge relay and connections. This small voltage drop will reduce the charging current for the habitation 12V battery. Hence better to connect more directly to the starter battery via suitable fuse and relay.
Where do those blue and white wires go to? Is it possible to trace them?