I've used flex seal on some things on my house. I would never use it again. The areas were subjected to constant UV (sun) and the flex seal broke down within a few months. I would use sealant made for RV use. In this case some DiCor non-self-leveling sealant.
It may have worked for others, but to me, it was a waste of money and time.
Ben and Doreen
Home Away From Home - 2017 PT Crusader 315RST
TV - 2008 F250 Lariat SuperCrew 4X4 6.4 Diesel
Never Enough Time Camping!!