Logyk
I'm no expert on parking up campervans, however some lessons I've learned parking up motorbikes and caravans.
Make sure it's well drained (your fresh water tank and pipes)
Try if you can to park it up on a dry day after you have washed, polished and dried it off. Give it a run round the block to dry off brake discs and pads (helps stop them from seizing)
Grease door hinges, runners etc.
Make sure it's got mostly a full tank of fuel as this I was told reduces the condensation that gathers on inside of an empty tank when left for long periods. Used to always do this with motorbikes left over winter.
If you can leave the handbrake off put it in gear and chock the wheels this saves your brake pads seizing on.
As it's a campervan, suggest you take out any supplies or items that would encourage wildlife (a friendly we mice family) from taking up residence.
As for the battery if you can't charge where it's parked up, then are you able (or a friend) to take out the battery occasionally take it home and charge it? The advice you got is correct, in that it will still slowly discharge if sitting how fast is dependant on how cold it is and how new or tired your battery is.
Hope some of this of help. Suspect others on this forum have more experience from parking up campers or motorhomes