Nationwide's standard cover is 31 days but you can pay to extend that up to 180 days.

Other than that I believe that 'Insure and Go' and 'Stay Sure' seem quite popular. I don't know the exact terms for them but could give you a good starting point for your research.

Hope that helps.

Try comfort Insurance - this is what's on their web site.

Some insurers charge extra for additional benefits – such as EU cover; with a Comfort Insurance policy, this comes as standard. The Comfort motorhome insurance policy even gives you unrestricted European cover (including optional breakdown cover) and our Green Card is valid for 365 days of the year – so you don’t have to cut your trip to Europe short or pop home for a week to stay covered.