I'm not a fan of a Ravenloft Campaign. It is best when it is intense and a punctuation - if you try to be intense all the time, it looses the intensity. I have used it for decades the same way - as an adventure that is part of a longer campaign. The PCs take an unexpected detour to Ravenloft as part of their story. I often sandwich it between two light hearted adventures in the campaign, making the contrast very strident.
When they go, the PCs rarely understand why they are really there. They often believe they're there to defeat an evil (often the Darklord), to protect someone, or to do something else active. In the end, they'll feel like they achieve those goals ... only to have the victory turn to ash as they realize that Ravenloft's powers are going to return things back to where they were (or make them worse). However, they will walk away with victories even though they were helpless to solve their original goals, usually in the form of information.
Example: There was a major event in my campaign world that took place a thousand years ago. It caused a disaster that shook the entire world and plunged it into chaos for the next 1000 years. However, exactly what took place is not clear. The only way to discover what really took place (sort of) is to find your way to the Shadowfell and into the Ravenloft Realm where the perpetrator of that event relives that day over and over and over and over, never able to find the courage to stop it. PCs have found themselves in this realm many times over the years. Some think they've time traveled and are there to stop the Apocalypse. Some know they're in Ravenloft, but they think they're there to save someone (or something) destroyed in the Apocalypse that followed. However, from a story perspective, every time PCs have come there, it was for them to witness what took place so that they can be armed with that information to use in their current storyline. That event has a lot of fallout and impacted dozens of major campaign events since the 80s, and when Ravenloft was introduced in the early 1990s, it gave me a perfect vehicle to convey that information in a dynamic way. It comes to play every couple years.