South to Spain, Somport tunnel or Irun?

Hi all, I have been following various threads about recommendations for routes through to Spain but have not really seen any comments about the specifics of either of the above crossings of the Pyrenees. I have driven the route via Irun on the toll motorways many times in car but never travelled by motorhome before so am looking for some specific advice if anyone can help.

I am travelling in April 2013 with a 4.2 tonne motorhome so was planning to stay off the toll motorways. We don't really want to go via Madrid because of past experience with robbers so plan to go via Zaragozza & stop there before going to Murcia. The question is do I go via the Somport tunnel, where I have never been before so have no idea of the roads etc, or go via Irun and keep off the tolls from Bayonne to San Sebastian before heading for Zaragozza on the A15 via Pamplona?

Or is the 121 from Irun to Pamplona a suitable road for me?

Having looked at the tolls for my size, class 3 I think, they would be very expensive but I am not sure what they charge on the fixed bits around Bayonne etc. Does anyone know this cost?

I am looking at overnight stops around Abberville, Tours then somewhere around Bordeaux and lastly at Zaragozza before arriving in Murcia.

Your help will be very much appreciated. Regards, Roy