Car hauler trailers

I have a question here, there's a lot of old travel trailers out there that people would pay you to take away. Are their frames strong enough to turn into a car trailer?
I would think it depends on what the main frame rails are made of. Most of the standard 7k lb car haulers I’ve been looking at have a 4” C channel frame. I would think if the travel trailer had that, you could reinforce it everywhere else