I think you are all misunderstanding the OP's question.
What he wants, is to hold the trailer straight behind the truck ONLY when he's backing up.
Answer's the same, though: It's not going to work.
Why? Because it will never be perfectly straight. If there's any sort of inconsistency in the driving surface, or even a gust of wind, the trailer will start to turn all on its own. You can't "hold" that with a rigid link. The forces will just increase until something breaks.
Short of trading up to one of those Fords with the trailer backing assistant, your only solution here is to learn how to back a trailer. There's nothing wrong with your tow vehicle or the trailers. The fact that the RV park owner had problems means nothing. They own the campground, they don't camp. You just need to figure out what works for you.
The biggest mistake I see people making is oversteering. To back up straight you only need SMALL corrections. Yet, they're sawing at the wheel like a lumberjack, flinging it from stop to stop.