As mentioned above try lubing it, squirt oil in the key slot. Use a light oil, even penetrating oil.
In addition to that try running the key in and out multiple times. It's possible that it could be gummed up in there and one of the pins is sticking.
Also, after oiling it, try tapping on the key cylinder with a small hammer or similar. This may dislodge a stuck pin if that's the problem.
Tap tap,tap, insert key multiple times tap tap tap, key in and out again.
Then try turning it. One stuck cylinder pin will prevent it from turning.
Fortunately, these door handles and lock systems are widely universal. Should be no problem finding a replacement if needed. Yours will likely fit my TT and the neighbors MH and everyone elses too.