Travelling without restriction

Can anyone tell me how practical it will to travel through Spain taking in Leon, Salamanca and Seville and then back up to Santander through Portugal without booking any sites in advance and staying off toll roads?  We are planing to travel Portsmouth to Santander in January and then Santander to Portsmouth in February, these crossings will be booked in advance but the  prebooking of sites is proving too restrictive.  I am looking for encouragement just to go for it but would also welcome any note of caution. We will be travelling in a motorhome.   Thanks