We are currently in US in British registered 4x4 camper check www.travel.uklinux.net
for pictures and info.
How much you thinking to spend on a vehicle ? I may sell or even rent out my truck as we are now based in Santa Fe for few months and then in late August flying to Thailand for 6 months.
As mine is UK registered it could be very easy for you to buy. Depends how much you're thinking to spend.
We've been travelling in our truck for 2 years, fully capable machine, will go anywhere, but nowhere quickly !
Anyhow, back to your question re insurance.
You should have no problem getting US insurance but when you apply they will require you to have an address here in the states. Any address will do, friend, hostel, hotel, couchsurf host etc etc.
Progressive insurance has an online site where you can complete all relevant details and they insure online and email you immediately cover note for you to print.
Few other points . .. USA is BIG real BIG ! distances are huge here, but that said there are some fantastic sites to see.
If you are thinking of going to National Parks then get the annual subscription (2 people for $80) or they charge you $25 dollars per park.
Fuel here is leaping up almost by the week, currently Diesel retails at about $4.25 per gallon (that's 3.75 litres) or Petrol runs at about $3.60 ish. (still even though here they complain about increases it's way cheaper than UK !)
All else is approximately half to two thirds of UK prices, eg here in Santa Fe (upmarket) decent meal costs about $15 to $20 (street food will run at about $5 for reasonable snack/small lunch).