Hi UKMark
I don't have a answer for your question However I am planning to do the exact same thing you are doing in reverse.
I have just purchased a Right Hand Drive Sprinter Motorhome through my friend in the Netherlands ( we cant own them there ).
It is now sitting in storage in Amsterdam waiting for summer in the UK. ( Mid May for 3 months )
I never thought about any Visa for UK or Mainland Europe In the past I have just gone although I only stay around 3 months or less.
This is the 4th small motorhome I have owned in Europe
Have a great vacation in the USA.
Aloha From Kauai Hawaii
Leon