I got a wireless camera/monitor off eBay, gray market probably, out of China for about $65. Works great! I power the camera off the rear running lights, so I have to turn on my head lights to power up the camera. To turn on the monitor, I wired it to a switched 12volts.
I'd be lost without the camera. My camper does not have a window in the rear door that would allow me to see out the back. I tow a very narrow trailer, one for a single wide jet ski modified to carry a kayak. My other trailer is for towing a motorcycle.
I'm able to back my trailer down a boat ramp and launch my kayak without anyone spotting for me. For $65 it's worth it to give it a try and see if it works for you.