Water coming in around door.

We went away last weekend and the Saturday it rained heavily. My coat was hanging on hook behind front seat and next to habitation door. Early evening we decided to go out and found my coat absolutely soaked through. Inside step had about inch deep of water. The only place we could see where rain had come from was where the black  rubber seal around door had perished a bit at the top.  It was fitting tightly all around door but just a little bit damaged at top. We mopped it all up and threw coat in the shower room. Could squeeze water out of it.  . Next day found more water on inside step, although no more rain. Water must have been held somewhere and seeping out, maybe out of seal. It seemed a lot of water for such little damage to door seal.As fast as we dried it more water appeared from edges of step. 
We want to replace the rubber seal first and hope it will solve problem, but where can we get the right seal? Can't see seal properly without pulling it off which would leave it more vulnerable.
Have Auto-Sleeper Broadway EL 2012.