Class A metal roof sealant

Clean and dry the joint real good first. Then cover it with a roll of Eternabond and you'll never have to seal it again. The stuff is great!
https://www.bestmaterials.com/Eternabond.aspx

I have a metal roof on my Monaco and I’ve done what gbopp has recommended. Eternabond on the seam. Just clean the dirt and you can apply the tape right over the existing chalking. They have a role which is 6 inches wide. I’ve had the coach 12 years and the eternabond has been on it for 7. Follow the directions and you can be done in a few hours

I've never owned a metal roof but, I would clean the seams and cover them with Eternabond.

For a metal roof, yes. I wouldn’t wait for cracking or whatever you wrote. At least yearly maintenance is essential. You’ve been lucky.