Are CL's o.k?

I am just starting to use cl's and have rung up and booked for 2 this year.Neither needed a deposit but just took my name.Having read somewhere-can't remember where as my brain is soggy, that on arrival people have been turned away as the site is fully occupied.How can I prevent this? I am going to ring to confirm a couple of days before we go but being disabled I have spent a lot of time finding a suitable cl.Also have seen and been involved in the leaving late on sites especially Sunday how does this apply on cl's.