There are some really good US road trips that you can do in under a couple of weeks if you don’t have time to drive across the whole country.
If you’re coming to the US for just a few weeks and flying in and out, no problem! You can enter under the Visa Waiver Program using an ESTA (electronic pre-approval).
It’ll definitely be cheaper to book your rental car or rental RV before you leave for the USA on an Australia, UK or New Zealand car hire website.
…and it’s probably more trouble than it’s worth unless you’re spending at least six months road tripping.
“Stuff” is cheap in America. Pack light or, better yet, come with an empty suitcase! Clothes, shoes, toiletries, electronics…it’s all cheap in the US.