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.