I bought a fancy camera/screen system to install into the trailer wiring (very similar to the one endlessclimb posted above), but my better half lost the camera so I never got to use it
instead of buying a whole new set, I opted for a baby monitor that plugged into a battery pack I already had. I used velcro command strips to stick the camera to the front wall of the trailer, that has been very secure. I have a window between the tack room and horse area, so just put the power cable through there. The battery pack has a handle and sits on the bridle hooks under the window. It’s certainly not a fancy setup, but it’s worked super well for my purposes. I already had the battery pack so it cost about $50 and I didn’t have to mess with the electrical in the trailer. (If you do go this route I’d recommend spending a bit more on a monitor. The screen on mine was messed up from the start and recently stopped working all together, so I need to get a new one)
This is the power pack I got, but anything with an AC adapter (wall plug) should work. The longest trip I’ve been on is 3 hrs round trip and it only used around 30% of the power if I’m remembering correctly. The benefit is that you can use it for other things too, like charging your phone or other devices. We do a lot of car camping so this has been a great investment regardless. After that trip, I got a USB cable for the camera, so I’m guessing it would use even less power now.
This might not work for your trailer, if I’m remembering correctly there isn’t a window or a door between the horse and tack area. If you’re ok with the idea of drilling holes in your brand new trailer (I know I would be hesitant!) you could probably drill a tiny hole in the top to pull the wire through. I don’t know enough about installing a proper camera to know if you would need to do that anyway? Just a thought. But thought I’d share what’s worked for me!
Eta for anyone debating whether or not to install a camera: DO IT! I cannot tell you you how much it’s reduced my stress levels to be able to check on my ponies back there. I have an irrational fear of them falling out the back of the trailer, or the floor falling out from under them, so being able to glance over (or have my copilot check on them) has reduced my stress immensely. I cannot recommend it enough.