If you can buy a quality 5th wheel a few years old and keep it in good mechanical repair and lots of wax jobs, it can last many, many years. There are differences in the manufacturing process. Some are assembled with staples. Some are done with good craftsmanship. Cabinets and drawers will give you an idea of quality or not. The roof fit needs to be perfect to protect from leaks.
However, you stated you will only use it during winter so it's not going to get a full-timer's heavy use.
I assume you'll go someplace warmer in winter. Why not consider a Park Model trailer that's set up in one of the many snowbirding areas? When you're ready to go, turn the key and you're out of there. You can purchase one much cheaper than you can a good quality 5th wheel and a truck to pull it.