Old wiring headache

highoctane500

Hi all,

I recently bought a 1990 Coachman 500/5 With a bodged up battery and charging system. I'm in the process of replacing it with a 20A charger/transformer. The control panel in the van has had more surgery than my nan, so that's coming out. My question is,
where can I find a wiring diagram for the van? I've looked everywhere online and even sent Coachman an email, with no luck. Also why is all the wiring so thick? It's got 2.5mm sq cables throughout. Even on the 5 amp circuits. If I replace this wiring is it
necessary to be so thick?

Thanks