They are usually glued on, so you could probably glue the broken piece back in. I've done that before myself. I use weatherstrip adhesive.
The shape of the seal will vary by the manufacturer of the rig, though they are all standard rubber seal products. You should be able to get a piece from a dealer that handles your brand of RV and it is certainly possible that another RV brand dealer will have a matching one also.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL