Make sure the coverages are the same, or if not the same, that they meet your needs. A standard automotive policy is inadequate for RV collision coverages - it will exclude many "house" items from coverage. But if it is an RV specific policy and its cheaper, go for it.
It may just be that your RV policy with Progressive is expensive to begin with. Shop around. We've found National Interstate to have the lowest rates and the best coverage for us, but you mileage may vary.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL