Do you definitely need a campervan? Odd question I know, but the US idea of a campervan is the massive RVs, which sleep 8 and have satellite telly. You will pay over the odds for it though. I got a ford aerostar people carrrier, and found that the seats fold down in it to form a double bed. Alternatively you could get one of the big estates or 4 x 4s, in which you could sleep comfortably. There are also loads of cheap/free campsites and cheap motels everywhere.
Buying can be a bit of a hassle if you dont have a US address and SS number, and you pay a lot for insurance if you dont have a US license, which is also difficult to get without the above.