Same with Katet, I found it was time and maturity.
We have a cottage, which prior to Brendan, we would visit a lot and spend weekends. When Brendan was a young infant, I rarely went because he had such a rough time settling down in strange places. BUT, when he was 5 months old, it improved, and we pretty much spent the whole summer up there. His ability to fall asleep independently was fully set by 5 months old.