Welcome to the RTA Forum!

Planning far in advance of a trip is a great thing, the planning can be almost as much fun as the travel itself!

Since you've got a full month, I'd at least think about doing a loop of the entire country. There are pros/cons of doing so, but if budget is a concern, returning the vehicle in a different location, no matter if you choose an RV or a car, will likely add severe hundred dollars or more to your bill. It would also make it easy to see two distinctly different parts of the country as you head across.

You mention this trip is 20 months away, so I'm assuming this would be a spring trip, and if so, that could work to your advantage to find a deal on a one way rental. Many times there are one way specials for car rental for people who take cars out of Arizona and Florida. For RV rentals, Cruise America is one of the largest rental companies and they have a depot in Phoenix, where you might have good look finding a special rate for a one way rental.

Being from the UK should have no impact on your ability to find a rental, and in fact, sometimes foreign tourists can find better rates for car rentals by going through a consolidator based in the UK/Europe.