I had this same problem a while back and posted about it. It dove me crazy for a year sealing every thing in site and it still leaked. If you remove the rail you will probably find the sealing tape in back of the rail which was not the problem. In my case the water was getting into the top channel where the vinyl cover is and running all the way forward to the front cap. The MH is parked on a slight incline. The water eventually rusted out some screws and some water enter through the holes. I replaced these screws with SS screws and recaulked the vinyl cover, and continue to check every year. I found the Geocell caulk to not crack and drys harder than the Dicor more adhesive like.
