In the Hardware store world the item you are looking for is called "Vulcam" which is a tubed Butyl caulk designed for roof flashing sealant use. Any roofer can tell you where to buy it as they all use it.
I like the Dicor Lap Sealant line
sold at RV places and it comes in 2 types.
One is a Self-Leveling that can only be used on flat horizontal surfaces as it is runny so it levels and the other is the same stuff but designed for vertical surfaces and somewhat less runny.
In my experience asking at every RV place I am in this is the stuff that they use.
Everywhere I have used some it has done exactly what is advertised,it just works.
Also it is white!