caulking

Dollar Tree Awsome cleaner and an old tooth brush.

I use a single edge razor blade to remove the caulking and then apply new caulking. You can use masking tape to make a nice edge when caulking. Just smooth out the caulking with a wet finger the remove the tape, Always use caulking approved for RVs. Not silicone.

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.