I have used Silicone sealant to finnish off the joins having applied a good amount of the proper caravan sealant in the gap first.
I was told this is important as the movement you get in all caravans means that ultimately the silicone sealant will split or come away from one of the sides of the join.
What is needed is a "non-setting" sealant to accomadate the creep that occurs in the joint. However, as the finnish on these sealants is never the nice bright white (or whatever colour you want) I run a thin finnishing strip of silicone over the top.
I get my sealant from my local caravan accessory shop but I am sure you could get similar from most hardware stores. I would recommend you talk to an expert first tho - as some plastics used in caravans react badly with the most innocuous things.
I remember one Post that stated Lsterene Mouthwash had ruined the sink in a new caravan!
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