Does AAA sell insurance in Hawaii? If so you might try them. After a bunch of places gave me the runaround AAA gave me liability coverage. Don't use their website -- it's not set up for quoting these types of vehicles. Call them.
I couldn't get RV insurance companies to bite because I don't have a permanently installed toilet. AAA was willing to insure it as an E-250 cargo van, even though it had a commercial VIN. They just sent someone by to make sure it didn't say "JOE'S PLUMBING SERVICE" on it or something.