In reply to Joshua Willett:
We did exactly the same a few years ago, and the specialist insurers wern't interested and those that did quote cost an absolute fortune.
We ended up insuring through NFU Mutual, who gave a very good quote based on photos I sent them of the conversion - very basic; gas burner (removable) fold out sleeping platforms / benches some storage, carpeting and insulation.
Give them a shout and see what they say. One key point is that the gas cylinders etc are not "fixed", i.e. they are secured but removable easily, as is the burner / grill.
NFU's whole attitude was great, after getting short shrift with other insurer. When I explained the mods, they laughed and said that compared to some farm vehicles they are asked to insure those mods were nothing.....
We have had to claim with them after a crash as well and again, their customer service and assistance at the time was superb. Including sorting out a replacement vehicle on a Sunday night on Skye so that we could get ourselves home.
Cheers,
Stuart