How many days do you mean by extended? Different insurers have different rules. We went with the caravan and motorhome club ( we are members) they look at a range of different providers to suit your requirements. If you search on here for insurance there is a range including Saga, Comfort, Adrian Flux and many more.
If you explain your situation people with similar experience may pop up.
We originally we were with the CCC club but I think I left because at the time I think they limited either the insurance or breakdown cover to 90 day trips (in practice or generally I can't see us being away that long but I don't see why suppliers should tell me how long my tours will last!). We moved to Comfort for both insurance and breakdown.
Our standard policy includes cover for up to 180 days abroad. BUT, we were able to negotiate extra time - 2 x 30 days for a few extra £, so we now have cover for 240 days abroad, which is exactly what we wanted.
We also have some other special conditions included - we garage the van in Spain for 6 months during winter (very few UK insurers, in fact no others I could find, would provide cover). We fly backwards and forwards to use the van during the cooler months.
And the price of cover was good. A helpful team who seem able to design cover for whatever you want.