Only time for a quick reply!
Don't use your van battery, many do without problem, but car batteries are not designed for constant discharge.
Get yourself a leisure battery, this is the kind of thing they are designed for.
At least an 85ah one.
You can get a thing called a split charge relay which will allow it to be charged up from the van, but you will still need to top it up at home.
I have a permanent lead out to the van, I have my inteligent charger strapped into the van and just plug it in overnight.
Effortless and no risk of ruining the vehical battery
Ian