Hi Joe,
The first thing which jumped out at me, and which you should know before you go any further, is that the normal Visa Waiver Program allows you a stay of 90 days. Have you actually checked with the embassy in London, or your local consulate, if you can stay for 105 days? On arrival, you will need proof (your ticket) that you will be leaving the States within 90 days.
You may like to do further research on this. As Michael said, you need to find a place to park it everynight, and those places typically will not be in big or little cities. You can't just pull over anywhere. That then means you will need taxis to get around. Only in places like NYC, DC, Chicago and San Francisco - and maybe a couple of others - can you rely on public transport. Depending on your mix of cities and nature, you may find that what you have been led to believe is not quite accurate.Living in an RV will prove much cheaper for us on the whole is what I have been lead to believe.
Lifey