When I lived in Moab, foreign licensed RVs were spotted. I would say looking back, most were Earthroamers. How they got here? Most likely via one of the many cargo ships that offer vehicles shipping (in and outside of containers). I would guess they would need the same safety features as US vehicles (not sure about air bags), but European vehicles tend to be as safe or safer than US vehicles.
Need to get US insurance, some kind of adapters for power connections, and possibly some type of inspection at the port? Not sure, might depend on what country the port is in.
European to here is probably not a size issue, but I have heard that US to Europe can be since many roads in many European countries are narrow and possibly too narrow for many of the large RVs seen in the USA.
I know a guy who shipped his Harley to quite a few countries so he could basically ride across the world on his HD. Needless to say, he depended on freighters for most shipping, but I believe for one jump he used a cargo plane.