We have ours insured with National Interstate through Explorer RV and cannot speak highly enough of them.
We had a claim two days before we were to return to Australia. Within 48 hours the claim was approved and the repairer had been paid. The roadside assistance guy who helped us the night of the problem, volunteered to pick up the 5er after it had been repaired and take it to the storage facility. He disconnected the batteries, filled up buckets of water and left it exactly as we would have ourselves.
I believe that the test of a high quality insurance company is how they react and treat you when you have a claim. National Interstate has my vote.