I expect that's just it. A motorcycle and a motorhome are vehicles that the insurance company see as recreational vehicles. They think you are only going to have them on the road occasionaly, and therefore see them as a lower risk. The Suburban and the Escort they see as cars that will get daily use, will be on the road nearly every day of the year, and have a higher probability of being involved in an accident.
I'm not saying that is or is not how you will use the vehicles, it's just how the insurance company thinks you're going to use them.
-S