As an alternative, avoid Madrid and go via Zaragoza with overnight at Zaragoza or Calatayud (Sabinan) then to Valencia, then south to Murcia region. Depending on how far you want to go the first day, then either of the zaragoza area sites, of Navajas or Moncofa near the coast just north of Valencia
If using Sabinan, a bit of map reading will get you there without actually going to Zaragoza.
I've used both routes to get to the Mojacar area south of Murcia and manage it with two overnight stops from St Malo, so you should be Ok with two from your ferry port.