RV camping sites book in advance

Hello, we are planning our 1st RV trip from Denver in Colorado and Utah during the 2nd and 3rd weeks of August. We are not sure of our trip in terms of number of stops and mileage. This is why we would like to keep some flexibility and not book everything in advance. Do you think it is feasible not to book in advance the RV camping at this time of the year or do you advice to book?
Many thanks for your advice.