This is what I use.
http://www.amazon.com/dp/B000R4UB08/...40_TE_3p_dp_i1
I've used it for years. Just ordered more the other day. Like all caulk, you have to dig out the old stuff and clean up around the area with rubbing alcohol before applying the new caulk.
Does it last for ever---no
Does it crack---yes
Does the stuff they put on the trailer at the factory last for ever---no
Does it crack---yes
On the roof I use Dicor lap sealant.