You are in for a challenging task.....
ATT Papworth are in liquidation now AFAIK
You may find an approach to St John Ambulance might help as it is just possible that they may still have a 2007 Mercedes Papworth conversion in their fleet......
The wiring (and plumbing for gas) is complex, yes it is based on the common rail engine but from then on all bets are off. Some were equipped with high capacity batteries and upgraded alternators, others not, some have 230v available at various points, others not at all. So it is VERY difficult to advise without much more detail including pictures.
I used to be a senior officer in SJA and am familiar with many of the quirks having had to repair problems with aspects such as blues and twos not working, suction machine not charging, interior lights or some external ones not working (often NOT a simple blown bulb) and many other quirks.
For fun I completely rebuilt the rear of an ex-service emergency vehicle, which is now happily being used in Kenya having been shipped out for a charity by the RAF on a Herc. It was stuffed before it left the UK !
If I can help I will happily but more details are essential. The Merc was originally designed so that after 5 years the back end could be lifted off and reused, but sadly they were not that good at surviving five years. They were also dreadful underpowered with a top speed of around 65 downhill with a following gale. The blues and twos were routinely switched off on motorways 'cos everything was going past.....
Thinking of ALL the front line staff working to try to help others;Doctors, nurses, paramedics, cleaners, NA's, catering staff, support services and all others. Their work is appreciated more than you can ever realise-keep strong and safe.
Any typo that I make means the errorists win.
Remember Jo Cox and what she stood for.
Any comments above are only MY OPINION
and should be read as that.