We went RoRo Southampton to California, again happy to recomend using Kinstone (thanks to V8 Bertha). If going RoRo your vehicle must be empty or they may not accept it. We bought cheap camping gear over there and flew in with as much of our personal stuff as we could carry.
Here is some information on shipping your vehicle or trips in general to the USA.
Wallenius Wilhelmsen Test Lane Southampton call Tony Wite 02380715223 to book your vehicle RoRo from Southampton to Port Huename, Ventura. Just above LA. California. We paid about £1100 outward trip, East Coast is cheaper.
You will need to contact US Customs and the Enviromental Protection Agency beforehand. you must have a letter from the EPA allowing you a temporary import. You can have it in the US for up to one year but it must leave the country when you do. (we had originally hoped to store it and return)
To book your passage back in the UK it is cheaper the USA end. use Ted Rausch co Phone 5624358231 Fax 5624377585 ask for Amando or Tanya.
Port Hueneme California is the best for Nevada , Arizona , Utah.
We flew in to Las vegas and got a one way hire to Oxnard Airport, stay at the Country Inn , Happy Hour and
Pool , but book well in advance as used by Military Officers Phone 8059865353 . From here it is a 5 minute walk to the Customs , Agent and Docks.
Only one Customs Officer so phone in advance for appointment to clear your vehicle . 8054888574 You will also need the agent to have Agriculture Dept. Clear for dirt ( very strict )Arrange Steam cleaning with UK agent at Docks before shipping. It took me half an hour to clear customs and be on my way.
Return shipping can only go from Long Beach nr Queen Mary.
Once in the US there is an abundance of fun and off road trals to be had, wild camping, reasonable hotels,mtels. Eating out is cheap and good quality. In short there is something for everyone here. I would recommend Moab in Utah, Northern Arizona is also stunning and the Rubicon and Dusy Ershim trails in Northern California is another option.
Hope some of this helps :-)