You must be able to present a valid passport or ID of all travellers at check-in. Sometimes these are kept for one or more days. Despite the many objections, many campsites still insist on this. A tip could be to bring your own copies of your identification documents with the ID numbers and the passport photo crossed out. On the copy you can then write down the date and for whom this copy is intended. There are also special cases for sale that ensure that the ID numbers are not visible. However, it remains a recurring fact that some campsites withhold your passport and/or ID as a kind of security deposit. You may want to try to give a deposit as a blockage on your credit card to prevent your passport/ID from being kept. You are responsible for having the right documents in your possession. Think of the necessary vignettes in the countries you travel through.