Rubber piece on gutter rail came off???

It would be a great time to pull all that insert and seal all screw penatrations...or atleast look for holes without screws. Thats a good spot for covered up shotty work o hide

after several years in the weather, the vinyl hardens and cracks, probably that's what happened to yours. I can always find a roll of it at my local dealer. I'm now replacing all the vinyl on my '05 Trail Cruiser, top rail and corner trim. Whenever I do that, I get new whitehead, square drive screws, coat them with caulk and replace each (usually) rusted screw with them.

Once the vinyl starts crackin and breaking I think water can follow the screw into the trailer.

The vinyl insert is cosmetic to cover the screw heads.

If you mean the vinyl insert covering the screw heads it probably just shrunk. It sits in the trim channel. You just bend it like you are going to fold it in half and it expands into the track. A dab of silicon or caulking at the end to hold it in place.
I noticed this shrinkage on the last two campers our family purchased. (2015/2017) They are building the new ones as cheap as possible. Previous 2007 went about 7 years before the trim started to shrink. It was twice as thick as the replacement I was able to find.

I think he means the plastic trim above and behind the spout. It is cheap, and like mentioned available online or RV parts places. It is mostly decorative to hide screws, but will help keep moisture from rusting screws, that could lead to water intrusion.

Jerry

Rubber piece? Are you perhaps talking about the plastic rain gutter spout? ... available anywhere RV parts are sold. Usually snap in place and secured with lap sealant, perhaps a screw if necessary.