Hi Dominic and welcome to The RV Forum.
We would like to spend a good amount of time doing this (at least 1 year, but probably a fair bit more) ...
Be aware that you'll only be allowed to stay in the US for 6 months at a time with a B1 or B2 visa, although there are some ways to extend that, and these are discussed in various topics on this message board. Most folks visit Canada or Mexico for a while, then re-enter the US and are granted a new 6 months stay.
Be sure to check out the article in our forum library entitled Guide for buying an RV in the USA
. There are also numerous other articles in our library that will help with your purchase.
How is the issue of insuring a vehicle handled in the US these days? I have read this can be a problem for foreigners, but not an insurmountable one, I assume?
The main issue is that, because you don't have a driving record in the US, your premium will receive a significant loading (UK insurance term), probably 100%.
I have a full PSV lience in the UK which may help.....?
Not really, because most RVs can be driven using a car license here, although a few states require an additional test for diesel coaches.