We did several things that raised insurance issues. Your circumstances will vary, but our experience might be informative.
These factors all affect the kind of insurance you can get or will need for a fifth wheel rig:
We were fortunate in finding an insurance policy designed just for fulltimers that covered all our needs in one policy (except for business insurance): RV Advantage. The price was much better than the total for the bunch of separate special purpose policies for which we had been getting quotes.
If you are living full time in your RV you should probably pay special attention to insurance for when you are parked. While towing, your trailer and contents are covered by (or should be covered by) your auto insurance. When parked, your trailer is a "house", and automotive insurance does not apply.
You need home owners insurance, just as you would for a house.
We had a whole house full of furniture and personal possessions in a storage unit the entire time we were on the road. Not all RV insurance policies will cover stored items, especially if you don't have a regular fixed-address house.
If you have a home office for a business in your trailer you may need special insurance. At the least you should have business insurance for any business equipment. But the fact that you have a home office in the trailer will cause some insurance agents to refuse to insure your trailer at all.
The folks at Farm & City Insurance checked with the underwriters of the RV Advantage insurance (i.e., National Interstate). As long as we were just running a home office and did NOT have customers or clients visiting the trailer on business, their fulltimer's homeowners insurance would cover the trailer for all the usual: fire, theft, personal liability, etc.
There are a variety of ways to categorize trucks, as described by Changin' Gears. But as far as insurance companies are concerned, light duty trucks are those in classes 1 through 3. For example, these are all light duty trucks (regardless of the company's advertising): Ford F150; Ford F350; GMC 3500; Dodge 3500. This is important because many insurance companys will only insure light duty trucks under a standard automotive policy. Some will not insure any truck other than a light duty truck.
The Ford F550 is a medium duty class 5 truck. GEICO would not cover it; I had to drop GEICO when we replaced our GMC 3500 with the Ford F550.
Some insurance companies want to charge commercial rates for a medium duty truck. Their reasoning is that it is a commercial vehicle and cannot be used for pleasure.