In my experience (last time was 10 years ago) of hiring in the US it costs the same per mile to run a RV of similar size there as here, unless it's changed recently they tend to run inefficient engines with RV's built like brick £$%^ houses. In fact last time we hired in US whilst it cost about same when cruising, putting your foot down just resulted in more noise, a huge increase in fuel use and very little extra mph.