I know this is an "older" post but thought I'd add a bit of new info.
As we have relocated to WA, we need to move our insurance from CO to WA as well. I am happy with the service that Farmers was giving me and they also work with Foremost Insurance for RVs. I walked into one of the local offices and asked if they knew much about the RV and full-time insurance that Foremost offered and they said yes and were kinds surprised that I knew Foremost offered full-timers insurance.
With my CO agent the F-450 was insured through Farmers and the Landmark was a "rider" on the policy through Foremost.
As we proceeded to start the WA policies, we found the Farmers would not cover the F-450 as a "private" vehicle, but could as a "personal commercial" vehicle.
As we investigated further we found the the Foremost policy for the Full-time fifth wheel required a "medium duty tow truck" to be added.
After several calls, we determined that Foremost would cover the F-450 and Landmark as a 2 vehicle package. On my policy paperwork there is the disclaimer "Your policy insures a medium-duty recreational tow truck. You'll see the term "motor home" used in place of medium-duty recreational tow truck."
We now have the F-450 insured with Foremost and have Foremost cards, but we get to keep the multi-vehicle discount for the Jeep even though it is the only car actually insured by Farmers.
I did check and all the uninsured motorist and other coverages are still valid for the trailer even with it is not hitched to the truck and we have additional live costs covered when the trailer is not available for our use.