Don't over think it. I have been camping since I was a kid and its much easier than most make it out to be. If your horse has good manners then just tie to the trailer. I have done this many times and it works fine. Just tie short and high after they eat so they don't get their feet tangled up or try and lay down.
Back 20 years ago, I did a lot of NATRC competitive trail rides. The rules were everybody had to to tie to the trailer at night. My horses did well with that for years. I'd give them a hay net tied up high enough that they could not paw it or get a foot caught in it.
After a few years I bought some HiTies. They were great since they buffer any rope tugs. Made sleeping int he trailer with a horse tied outside a WHOLE lot easier. You didn't feel that constant tugging of the horse moving at night.
And Utah also has lots of places with no trees.
When possible I always high line at night. It gets the horses away from my trailer, so I sleep better.
I like to set the highline up where I can see it from the trailer windows. This allows me to look out at night and check on the horses
Some camp grounds have hitchen rails that I've used and my horses have done well with those
I frequently hobble my horses and let them graze during daylight hours. I've never trusted leaving my horses hobbled all night. My horses can run faster than I can even when they are hobbled. I've seen hobbled horses heading back down a trail and an hour later some camper looking for lost horses. They can hobble all the way back to the trailhead.
Some trail heads have corrals that are available on a first come basis.
And I frequently pull a hot string around a meadow and turn my horses out to graze. But again, I only do this in the daylight. I've had too many deer/elk run thru a meadow at night and knock down any hot wires
As you can see, there are all kinds of ways to contain your horses while camping, Your horses should be comfortable with all of these methods and allow you to choose the one that works best for the camp you are at.
Here's what I'd do. Get some good steel posts 5' long along with insulators that work with the posts. A sledge hammer will drive them in. My pen would be about 30' long by a min of 12' wide. The charger would work off the truck battery. Two strands, a hot and neutral will contain even a restless horse. These dimension allow the horse to move about. They smell things we don't so I want him to know he's got moving around room.
Too often I am camped 7-10 miles away from my truck, So I don't want to carry heavy items. So I keep some simple fiberglass rods in my trailer. I can throw the bundle of 20-30 rods on my pack horse and fence off an entire meadow. But I could just as easily set up the 12x24 pen that saddlebag describes. Usually meadows are soft enough earth that I can just push the rods into the ground. Trailheads, Campground etc often have more rock in the soil and you would need a T-post that you could drive in.
But the problem still exist that wild game animals don't know what a hot wire is, and the chances are that when they touch it, they may spook thru the fence vs backing away through it.
Here is a moose at the trailhead eating somebody's baled hay out of their truck.
I've been awaken at night by noisy horses and when I went out of my tent/trailer I found a moose eating my horses hay and my horses backe dup as far as their leads would allow.
I use 3 chain to gether panels, using the trailer as the fourth "wall" works fine. When I am camping away from the trailer I use a hi-line as I live in east Texas with no tree shortage. However since I will soon be living full time, in the DFW area with no freaking trees of any size, I imagine we will introduce the picket pin, he hobbles so it shouldnt be a difficult adjustment.
I have panels and used those for years when I did endurance. I had a mare that I trusted and she was fine in them. I've seen horses that got their heads thru a panel, panicked and took off with the panels still attached to their head!!
I've also used electric. One good thing about electric, if something really spooks them, they will just break the wire. But with the electric, you can make it as big as you want :) So they have plenty of space to roll or lie down etc... Mine has a battery pack. Doesn't take them long to get used to it - set it up at home. I don't like tying to the trailer just because I would like it long enough for them to lie down if they want, and I'm afraid of it getting around their head but depends on the horse. Alot of people still do it and it works.
I agree with huntfishnride. Don't over-think it. People are going to have a horror story for anything you decide to do. If this is just an overnighter, or even two nights, just tie to the trailer. Water the horse in the evening after feeding and first thing in the morning. Don't leave the bucket there where they can reach it or they'll kick it around or play with it and you won't sleep. Same for letting the horse stay in the trailer overnight. You won't sleep and you'll get complaints from your neighbors. Horses in a trailer are NOISY. If you try to picket in the NM desert, you'll end up having to get up in the middle of the night to untangle his picket line from the brush.
Tie to the side of the trailer and make sure the lead is long enough for the horse to reach the ground with its muzzle, but not long enough to step a foot over it. We've even had our horses lay down next to the trailer, after a hard day's ride, with no problems. If there is more than one horse, tie them both on one side, or at least where they can see each other, but not close enough for them to get into each other's feed.
Then, if you decide this is something you enjoy and will do often, start collecting stuff like an electric corral or lightweight steel panels. Those things are not cheap and aren't worth the expense for one-nighter.
Speaking of laying down by the trailer. This mare laid down just after I got her packed, She had not even done any work yet!
Thanks for the photos and tips! I'm new to the board (I've snuck around and read posts for a while..just now writing on threads ) I haven't done any camping with my ornery gelding yet, but hope to once the winter breaks. Thanks again!