We found the same problem a couple of years back when we started to do our trips of 3-4 months at a time.
As pointed out most household insurance will give you 30/30+ days unoccupied cover as standard, some others will give 60 days and some of the specialist will give you 90’s days (but at a premium).
We found that the Camping and Caravan Club (Club Care Insurance) where one of the providers that provide 60’s days unoccupied cover as a standard, however if you were a camping and caravan club member this was automatically increased at no extra cost to 180 days. Coast of membership is £39 for a year for online membership (covers the who family).
Doing the comparisons at the time the CC deal was by far the easiest and cheapest way to get the level of insurance that we wanted. (I appreciate that all these products change. I have been happy to stay with the CC product – though would be interested in others experience from their research in to this topic).
Clippie