I agree with this. 楽しかった旅行です is NG and I personally never have heard or said it, but it is not because this line violates the grammar rule; (adjvective-past)(noun)("desu") is grammatically possible, e.g. "母と暮らした子供の頃、それが私のいちばん楽しかった思い出です“.
楽しかった旅行です is like "It is a trip which was happy" while
楽しい旅行でした is like "It was a happy trip". No doubt, the latter is natural and usual. You should use the usual one if without any special reason. This is the reason I think the latter is a better one.
**** And it makes me wonder,
Is it OK in Korean to say "楽しかった旅行です" like "즐거웠 던 여행입니다(?)" ?