The only time I've ever been overseas is this one time.
The only trip I've made to a foreign country is this one occasion.
この場合、次のような言い方ができますよ。
ーThe only time I've ever been overseas is this one time.
「海外に行ったのはこの一回きりです」
ーThe only trip I've made to a foreign country is this one occasion.
「海外旅行に行ったのはこの一回きりです」
ご参考まで!