I use Dicor Self-Leveling Lap Sealant.
Try to get as much of the old stuff off as you can then clean it real well with isopropyl alcohol. The Dicor will cover everything and self level. Good Stuff. The other option is Eternabond Tape, never tried it myself but heard it’s pretty good. – http://www.eternabond.com/RV-Leak-Repair-Products-s/22.htm