Leaving a caravan unattended for a week in France

We had akready booked a week on a barge with friends on the canals in France  when we decided to buy a caravan. We now plan to be in France for 5 weeks late April/May moving around except when we are on the barge. Do we need to pre-book a site, to leave the van unattended?  Has anybody any experience of this? Will it be a problem or can we turn up and negotiate?