There are other good options, but I use dicor lap sealant for horizontal surfaces (like the roof) because it is designed to settle and spread out a little bit.
On the sides I really like ProFlexRV... it's sticky and stays where you put it.
For seams between surfaces that are out of sight (like a roof) I use Eternabond tape. It's more visible, but it's far more permanent.
Just make sure you have clean surfaces!
I will say that whatever you use, if things are splitting and cracking you will need to stay on it. Check the problem areas twice a year and check everything at least once a year. Visual checks aren't too difficult and preventative maintenance is the best way to keep the water out.