Thanks for the reply!
So it would be a dry sauna which is quite a bit different to a steam room (less steam and moisture).
My plan was to create a sufficient barrier between the sauna area and living quarters and to basically make the sauna area a “wet room”. There are a lot of plans online for outdoor sauna builds which require similar amounts of insulation as I think the van will need.
Just to point out this will not be a permanent living space, just a project for weekend/camping trips, however the sauna will be used quite often parked in my driveway (another option could be to power the sauna through my home when it is parked there).
My main issue is getting my head around the electrics and how to calculate how much I need. To be honest its all a bit over my head at the moment.
Ive found a calculator online that tells you how many amp hours (shown in pictures). Would it work having a second battery just for the sauna? Is 384ah a lot??! I bet I sound like a complete noob 
Ive also added the specifications of the sauna, just to check if i’
ve calculated it right!