What do I do when I have a job related injury which has occured over time with no known actual date of injury?

I have rotary cup tears in both of my shoulders and a bicep tear in my left bicep. My employer is telling me the workers comp insurance will give me problems because this is an injury that has occured(possibly to my age) and over a period of time, and I have no documentation of injury. My boss is aware of this problem, he was present when I slipped and fell on an icy spot on a roof, he is also aware of the fact that I did trip and fall on a drop cord, because he saw me walking strange and asked about it. I asked him about the workers comp claim, gave him documentation of what doctors found and have not heard from him regarding this matter. Left shoulder has been a problem for a couple of years and is in the worst shape. My job requires heavy lifting, working above my head, and holding heavy objects in place until they can be secured.