Here's a good place to begin your research:
http://www.cruiseamerica.com/rent/one-way-rv-rentals/Be ready to pay significant money for a 3 week RV rental, and probably a hefty premium for dropping it off in a different location (one way rental). A rental for 6 adults makes it even more expensive, because you'll need a larger RV. you're looking at over $4,000 + fuel even to drop it off where you pick it up!
The size of the unit will likely make it difficult to get in and out of any cities and force you to rent a car, take public transportation, or taxis to get around.
It sounds alike a terrific adventure, as long as the cost doesn't become an issue.
