The deal is, I don't have a truck. This fifth wheel is temporary housing while our house is built. The jack I mentioned has a weight-bearing capacity of 5,000 pounds which led me to believe it could support the king pin ( estimated dry hitch weight is about 1,300 pounds )
Even if it can't "lift" the king pin, if it can support it temporarily that would be good enough. Or I just can just hand somebody else in the RV park $50 to bring their truck over for 15 minutes while I adjust things.