talk to an autoelectric guy
i used to do that here in the states if you had a ford or a GM id know just what to give u lol

you would probably want to swap the batteries out with deep cycle for starters
and hook them up in parallel (12v, double amps)

then take the alternator to your local auto electric and see if they offer an ambulance alternator (if trafics are used for ambulances, they dont sell renaults in the US anymore, so ive never seen a trafic)
i own and operate out of a Ford E350 superduty 5.4 liter V8 (american bawls lol) which are also used for ambulances here (custom box on a stripped chassis and cab)

the proper name for the converter is an "inverter"
the alternators used in ambulances here are good for well over 200 amps, where my standard alt is around 100 or a tad more

if thats available for your van thats what youll need
but talk to the autoelectric guy 1st he may have set up a system like you want before

my van:
img339.imageshack. us/img339/4854/montagemediumsb7.jpg