EASA has confirms in their website that CAMO EASA Part M will not be required for San Marino Registered aircraft operated by European Operators, find below extract and link to their website:
European Implementing rules for continuing airworthiness (EU) 2014/1321 do not apply to European NCC operators for third-country registered aircraft unless the regulatory oversight has been delegated to an EU Member State.
In particular, tasks related to the continuing airworthiness management do not need to be implemented by a CAMO in the sense of Regulation (EU) No 1321/2014. However, an organisation for the continuing airworthiness management is mandatory in accordance with point 8 (g) of Annex IV of the Regulation (EC) 216/2008.
In short, European NCC operators of third-country registered aircraft need to ensure that an organisation is managing the continuing airworthiness of their aircraft and that this organisation is able to demonstrate that the aircraft comply with the continuing airworthiness requirements contained in Appendix IV of Regulation 216/2008. This organisation does not need to be a CAMO; it can be the operator itself or any other organisation or natural person under the responsibility of the operator.
http://easa.europa.eu/the-agency/faqs/regulations#category-part-ncc-nco