RV insurance rates and motor home insurance rates will vary tremendously according to your state of registry and even your domicile. You will pay a lot more to insure the same RV in New York City, Los Angeles or Detroit than you would in Montana, or rural Texas or Arkansas. Think traffic volume and crime rates and you'll get the picture in a hurry. How that makes sense on a vehicle that is admittedly going to travel all over creation, remains a mystery to all but those infallible underwriters.
Why go to a "Specialized Company": Well would you get your annual medical at a dentist or eat steak at a sea food restaurant? Yeah, I guess you could but you would be better served at a diagnostician and a steak house. Like all business, insurance companies are made up of people and they tend to develop more of a comfort level in a limited field than broad generalists. It's like buying your R.V., you could get one from the local used car lot or you can deal with a major R.V. dealer, you just know who can best answer your questions.
The same will apply to the insurance company staff. While the insurance industry has a host of "general" companies offering so called "All Lines", R.V.'s would fall in the same category as trucking, marine, aviation, transportation, jewellers blocks, furriers bonds etc. All these lines gave rise to a small number of specialized companies that limit their marketing to their chosen field and accordingly develop staff expertise closely related to these fields, allowing them to deal much more expeditiously with their clients. (Well you asked!!)
There is major differences to covering a Class "C" and a Fiver. One is a self propelled motorized vehicle while the other is an attachment to a motor vehicle. Coverage for physical damage, read fire, theft, collision, upset, vandalism, windstorm, in short any damage to the R.V. itself, would be covered in very similar fashion on both units. Where the big difference comes in is in the Liability for Bodily Injury or Property Damage to others that the "MOTOR" vehicle might occasion. The class "C"'s premium will be all inclusive and require only one policy. The fiver will require it's own policy that will also afford liability coverage for injuries or damage to property of others but that is meant to cover the unit when it is stationary!!! Think slip and falls, dog bites, theft etc. With the fiver, you will also have a standard auto policy to cover your truck, and it will cover all "rolling" liability issues. (Confused yet?)
R.V. Insurance will normally be written on a replacement cost basis, in other words, no depreciation if your unit is destroyed by whatever! I understand that coverage for Additional Living Expense is offered by some companies and would be a must for full timers. That is likely something that non-specialized companies would not offer.