I am a newbie and thanks for accepting me to the forum.
This question relates to my brothers 2003 Ducato 520 motorhome which has a genuine 45000 on the clock, he does not use the internet.
Problem is when he hooks up external 240 volt source at campsite the power does not go through ? the inverter behind the drivers seat. Had previously been working fine.
I checked and the power does come in OK but not out the other end of the unit to the sockets in the rear department.
Hoping there is a 'mains' fuse somewhere which I have failed to see and fearing the said unit on the floor behind the drivers seat is faulty requiring replacement.
Any suggestions will be greatly appreciated.

Hi and welcome to the forum,

The FF Ducato forum is aimed at the base vehicle and your query is related to the habitation area, nevertheless many members do have motorhomes so you may get a response.

However the first issue is to clarify your query! An 'inverter' converts a 12V DC input from the habitation battery to a 240V AC output which is fed to a conventional three pin socket usually built into the inverter case (some vans can have a changeover relay so that this supply can be linked to all the sockets) Therefore the EHU supply doesn't need to be connected.

From your post it seems you are experiencing issues with the Power Supply Unit (PSU) which is fitted to most motorhomes (inverters are optional!) and does require EHU to charge the habitation battery and operate the 12V functions and the mains powered sockets and appliances.

THe PSU may have some external fuses and MCB's which can be checked and may have an illuminated on/off switch but if these are all ok and there are still no 12V and 240V outputs then it may well have an internal fault.

To end on a cuationary note, if you are not conversant with mains electricity please get an expert to look at the van, the 240 electrics can be just as lethal as the stuff indoors!

As the OP himself appears to be unfamiliar with MH electrics, perhaps he/his brother should consider buying or borrowing this.
https://www.amazon.co.uk/Motorcaravan-Manual-Choosing-Using-Maintaining/dp/0857331248