Think that would be true only for liability and not full coverage to the trailer.
I bought my TT in 2009 and insured it with USAA / Progressive. Dragging it to the front of the dealers lot, I had a little confrontation with the dealer's financing coordinator's suv. Called USAA to cover the damage to her vehicle (liability) but didn't bother to call progressive to fix the very minor dent in the TT. I now have a MH insured through Good Sams)/ National General for both liability (while removed from storage) and comprehensive while in storage and traveling. Turn in the tag when putting it in storage and getting a new tag after paying for the liability insurance prior to leaving storage. Saving a bundle!!! I don't know of any other rv insurance with a similar discount while in storage.
Nationwide. Originally they just insured the MH but they were able to save me a hundred dollars or so when I put the homeowners and our two cars with them as well. I thought they were easy to work with and they appear to have pretty good customer service reviews.