You can set a default year round minimum rental duration as well as for specific times of year. Many Advertisers opt to allow shorter rental durations outside of peak season and slightly longer ones at the busiest times of the year.
To customise these do the following:
1. Go to 'My motorhome'
2. Scroll down till you see 'Booking' on the left hand side and click on that
3. Once in 'booking' you can see the blue 'Add a minimum rental period' button. From there you can select dates and the minimum amount of days.