Welcome to the SOC!
We'll need some more information, to be able to help. Do you have any pictures of it? On the later Sunline campers, they put a tag with the model number right next to the entry door. All Sunlines came from the factory, with a sticker inside a cabinet. Here's a pic, from one of our previous Sunlines:
Here's one, from an older Sunline:
There are also VIN stickers, that were attached to the driver's side front corner:
Once you find the information we need, we will be better able to help. The holding tanks Sunline used were not the largest in the industry, but they do quite nicely.