Jack
I don't have a cargo carrier but I'm considering one for my Roadtrek 190P. Quite a few RT owners have the StowAway box that comes in two sizes. A few weeks ago, I camped next to a Pleasureway owner who had a Rola. These cargo boxes can be equipped with a swing-away hitch so you have access to the rear door. If your class B carries the spare tire on the back, you will have to use a hitch extender to clear the tire. I think the Stowaway box can be ordered with a longer hitch. The web sites for both boxes is below.
StowAwayRola cargo carriersJim D
2008 Roadtrek 190P