This is what you need:
Water ThiefYou just stick it on the un-threaded end of the faucet and hook your hose to it, long enough to fill the tank.
Most campgrounds without hookups do not have threaded spigots, to keep people from hooking up and hogging the spigot.
It is NOT intended to be used for hooking up for continual use. Un-threaded spigots (older ones, anyway) may not have back-flow prevention, so don't use them in that manner. Just pull up close enough to fill your tank and then move on.
Since this is at Sibley Lake, there should not be limits on the amount of water you can use at a faucet, as there might be in a water-scarce area.
Or you could fill up 5 miles away.
I'm envious, wish I were on the way to Sibley Lake, too!
Single empty-nester in Middle TN, sometimes with a friend or grandchild on board