I've had this issue and have also experimented with a number of products. I've found the best one being:
Motsenbockers Lift Off Spray Foam and Silicone Caulk Remover
Here are some others:
Un Du Adhesive Remover
Rubbing Compound
Mineral Spirits
What is happening is that the caulk is braking down. It turns dark and tends to spread out. Using the Lift Off stuff I basically clean away the top layer of caulk. I usually take the caulk gun to my rig once a year and touch up areas that have degraded to guard against leaks.