Also a fan of Eternabond.
Splitting after six years isn't all that unusual, especially if you have a certain travel route or favorite parking spot that racks your RV in such a way that the roof twists occasionally. That will open seams easily over time.
Agree that you should get a second opinion.
You could also consider going to Mexico. The RV park you stay at would know someone who could do the job and it would probably be under $100. I had the edges of my Winnie fiberglass roof done with EBond by 2 guys, cost was $30 labor and $120 total. Took them all of 45 minutes. I did the end caps myself which doesn't look as good as their work.