First of all, I don't think you have anything to worry about, unless you find damage under the paint. That would be the first step, wire brush, sand, and re-paint the whole tongue.
Here's how to lock the tongue:
I had a hard time figuring out how to lock it too. I had to figure out how the coupler worked first. When you pick up on the lever to pull it back, watch the hinge point. It doesn't stay in one place; it slides back. Next, try holding the hinge point from moving and pick up. Grab a ball and try to fit it in. It won't fit in unless the piece slides back.
Then I noticed a hole on the one side. At first, I didn't realize it did anything, but then I realized that blocking this hole is basically the lock. I kept a regular padlock through that hole for storage as one of those pin lock things wouldn't fit. However, I couldn't leave this on for travel as I couldn't make much of a right turn. So, I used a simple bolt and wing nut through that hole. I held the nut on the inside and screwed the bolt in from the outside. Depending on the bolt you use, you may have to put some washers against the bolt head so the latch can't go over it. Unfortunately, I don't have a pic of the bolt.
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