Siliconized latex caulk is not the caulk to use. I'd suggest to buy some 3M caulk remover and remove the Alex Plus. That will also allow you to see if the fiberglass is cracked (a real possibility). If the fiberglass is cracked, you will want to patch that with resin and fiber or just resin. Caulk will not hold it together for long.
The best caulk to use on the exterior is one of the polymer sealants made by Geocel. They are engineered for the transportation industry. Tripolymer 2300 adhesive and Proflex RV are two types of sealant made by Geocel.