First of all, when you describe it as a baby camper, can you stand up in it? If not, then you have a pretty rare Sunline.
The serial number should be stamped into the left side of the front coupler, or right behind it. It may be a little hard to read due to rust or paint layers, so be prepared to scrape a little. It should be a pretty deep stamping. It's going to be likely 5-7 digits, mostly likely 6, all numbers. I call it a serial number because the proper 17 digit VIN format did not come out until 1980, which Sunline started using half way through that model year.
In many cases, the serial number tag is under the entry door threshold, but it may not be visible in your case. The newer ones are. I think these older ones have it tucked under the door frame, so you'd have to remove the door frame from the trailer to find it. That's a last resort, if it isn't on the frame. Sometimes people have new couplers welded on the frame that cover up the serial number, so under the door is the only option.
If you're lucky enough to still have it, your trailer originally came with a piece of paper that was a quality control certification, adhered to the wall inside the largest cabinet or in the toilet closet. If you have this, the serial number was written on there.
If you can attach some pictures, I can confirm the model and year for you. The brochure can help too.
Like John said, replacing the glass in your Hehr windows is a better option than full replacement. A glass shop that would replace your windshield is a great place to start for ordering this. Hopefully the cost isn't too high since these are small panels of glass. While you could do it with the windows on the trailer, I think removing them is a better option since the seal tape is likely original. Clean up the surfaces of all old tape and re-install with new RV butyl tape to ensure a good seal. Finding new windows isn't a great option because almost all RV windows now are radius corner, so you'd have to cut the window holes bigger or fill in the holes in the corners, which wouldn't look good. Entire new windows, plus screens and inner trim rings, wouldn't be a cheap option either.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR