Not inexpensive, but you might try
Rearviewsafety.com for all your other needs.
They sell back up camera systems for commercial and non commercial vehicles and RV's that have multiple cameras.
They also have compact DC powered DVR's available that use both hardwired and wifi cameras. You can connect or link from 4 to 8 cameras. They use 1TB and 2TB Western Digital hard drives and also use SD cards. I believe they can be connected via cable to a lap top or computer.
I haven't fully looked into them, but will be checking them out closer as I get closer to becoming a single full time nomad. Besides considering using them for a trailer backup and security system, I will also have them installed in my tow vehicle for security incase of any bad encounters on the road.