Some companies do not underwrite items like that under a specific policy.
Take the Hartford. I have them and they covered all of my vehicles including the motorhome with liablity, collision/comprehensive on the MH and Truck, but not my POS commuter car, and last agreed value replacement on the MH. When I moved to the 5th wheel, they cover the 5th wheel with comprehensive and collision as well as agreed value replacement. The difference now is that liability is not required on the trailer, even if I carried the liability rider, if there where an accident the vehicle the claim would go against would be the motor vehicle (the truck.) However, we did look over my liability limits to ensure that I had the coverage I needed in the case of an accident totalling everything no matter who may be found at fault (or none) by the lawyers.
2006 Ford F350 6.0
2014 Primetime Sanibel 3600
Enough other vehicles to not bother listing.
Previous RV: 2001 Monaco Knight