Girlfriend gets 90 days out of the last 180 in the schengen area. Not a deal breaker but you need to spend time outside the schengen.
You are more complicated. If you arrive and stay on your US passport, you have the same rules. If you come on your EU passport, you could trigger residency if you stay too long. Also, you would not be eligible for deferred import rules.
As a non resident, you can bring the RV in for up to 18months without paying VAT. Leave for a day(truck), and you get another 18 months. As a resident, VAT is due upon entering. Typically north of 20% of what they assess as the value.
Buy simcards, don't mess with US cell plans. They are cheap. Got a 50gb in Italy last month for 25euro. Then get a Skype number and you can call us numbers cheap.
The other stuff is pretty easy...though insurance may be tricky as you won't have a local address.
