Just wondering how you know its a Forest River?
Does your trailer have a VIN? Maybe in a kitchen cupboard? If so, you might want to call Forest River directly. Here's a thread
with various numbers.
If its tank sizes you are looking for, then you could estimate it by taking the measurements of the trailer and matching with an equivalent FR floor plan and look up the tank sizes they are currently using. But not sure you need to do that. I'm guessing that you could put in whatever fresh, black and gray water sizes you want as long as they fit.