We are RH drive Iveco 4x4 and have come from Europe via Mongolia to arrive in USA back last November.
Having travelled for a few years in various countries I now have drawn the opinion that US is the most awkward we've come across so far. (for people not vehicles).
We arrived Long Beach having previously arranged insurance, 12 month temp import issued with no problem and we were able to drive vehicle from container without any US official ever seeing it.
We enetered utilising the '90 day, Visa Waiver Program' rules (ie no visa reqd.) again no problems.
We have spent our 90 days in and around Southern California and New Mexico. Stopped once (too slow on interstate!) but nobody has asked to see any paperwork for vehicle.
We crossed border in Mexico couple of weeks back, no problems getting in whatsoever and we saw nobody on the US side of the border (so technically we did not officially exit USA, you are required to find a suitable member of personnel to hand your green immigration form back to, we saw no-one, nobody atall, first officials we met were the Mexican border guards.)
Our plan is to re-enter the usa and spend Spring and Summer touring on our way up to Canada.
But, and here's the issue . . . .. . . . The VWP allows a 90 day tourist visit, but you should thenexit country for at least 90 days before trying to re-enter again. The problem is the US immigration/border control do not class entering Canada or entering Mexico as exiting their country. In other words your visa does not stop just because you enter Mexico or Canada, so if it does not stop then technically they are allowed to refuse entry when you attempt to come back. And that is our potential problem.
A fix for this issue is you could apply for a standard US tourist visa which would allow entry for 6 months out of any 12, this kinda helps but doesn't totally solve the problem if (like us) you plan 6 months in US then 4 to 6 months Canada and then back into US. because again the issue of only going into Canada does not class as exiting USA.
BIZARRE eh !
However, in reality despite the rules and regs I doubt there will be any major problems for a British passport entering US in an obvious tourist vehicle with proof of a tour (check the passport).
My advice is to cross at a quieter border point and not one of the main throughfares. theory being more sympathetic guards.
With regards getting vehicle in. The temporary import is easy to obtain, we were told that though it is valid for up to 12 months if we exit and then re-enter we would need to re-apply. It is just a paper exercise, doesn't involve any check of vehicle as such, does not cost anything. But if your vehicle is valued at more than $2000 it may fall into the next category which can be stricter ( I believe). So obviously regardless of your actual vehicle value for the sake of import keep it less than $2000.
Mexico - if you are coming through mainland mexico you will require to pay a vehicle deposit (usually about $300 and via your credit card), this is normal and is refunded (or not taken if credit card used) upon vehicle exit from country.
INSURANCE - Mexico - you will require insurance, they will ask for it at point of entry. We have had various military checkpoints in the country but nobody has asked to see insurance documents.
We purchased out insurance online via "Discover Baja" ( Discover Baja Travel Club - Mexican Auto Insurance
) it cost a total of $170 for 1 years liability only cover.
INSURANCE - USA - possibly you could get away with not buying, we haven't needed it yet so who knows??? Again we purchased online and cheapest we found was using an agent in SanDiego who arranged insurance through company called 'Progressive'. We searched around to get the best price for this one as the cost fluctuated wildly. I highly recommend using kimberley at 5th avenue insurance (tell her the english guy phil sent you !) 5th Avenue Insurance
or telephone 619-955-1261. Again liability only and it cost us $310
They will require an address in US to send documents (we used hostel at Hermossa beach).
Both insurances are for British registered, tourist, temp import vehicle.
Hope this all helps, sorry bit long winded !