We travel with two cats for the past ten years. They love the camping and are starting to get used to the 'catch and release' requirements of most parks. They want to be outside but we didn't want them to run free. It took a while but we now have a great system for leashing them.
We run a 'highline' from the trailer to a convenient tree.
1-I toss a weighted line over a branch, have the line drop down to easy reach then put a slip knot in the line and attach the end of the line to the slipknot. That way when the line is loosened the knot will drop down to my level rather than be drawn up to the branch and get stuck there.
2-I run the other end of the line back to the trailer and attach in under tension to the rig. This results in a rope run higher than head height across your campsite.
3-This is the key-we hook retractable leashes to the highline via clip that can slide up and down the line. We fix the stop point or a pivot point with a clamp on the line on either side of the clip. The other end of the leash is clipped to the cat harness (duh)
What makes this set up great is the leash is always under tension and as a result doesn't drag on the ground and get caught on everything. We can add extensions to the 25' leashes if we have a big enough site or if we are in a small site we have 15' leashes.
We use a variety of spring clips like on the end of a leash for weights or connectors for two leashes etc.
like I said this system has worked very well for us for years, we get very few complaints about tying to trees(I have a long tent pole I can use if necessary) We've never been asked by rangers about the leash length because they are so amazed to see cats camping.