This is a common problem in the Monaco/Holiday Rambler products. Serious water intrusion can result.
Even though it is high on the coach, that joint is often called the "beltrail."
The folks at Talin Manufacturing in Brooksville, Florida have a lot of experience with this problem and its repair. Also, they have a fix for the slide-floor rot problem that afflicts some Monaco/HR coaches. Their Web site is here
. Give them a call for a consultation.
In the Summer of 2020 I had to repair the passenger side of our coach's beltrail, followed by floor rot repair inside the coach. It was, for me, about as deep into fixing things on the coach as I cared to go. I spoke with the Talin folks several times during the repair process. They were very helpful.
On another topic: I have wiring diagrams for your coach. If you send me a private message that includes your e-mail address, I'll send the diagrams to you. Don't "publish" your e-mail address in public here, or the spammers will get it. Instructions on sending a private message are here