I have been looking into this for a while. I have a Basecamp on order and would like to have a security camera for when it's stored in a gated self storage lot. (A little different than your request but still related).
Yes the Nest and most home security cameras need Wifi. Panasonic just came out with a camera called Nubo but it's only in Europe right now. Should release here soon. It works on 4G so you can use it without wifi and it also works for some time on batteries. That could work as a backup camera and security camera.
Another option for backup camera is to use an old discarded iPhone as a camera that you can view from your newer iPhone. There are apps to hook them up. AtHome is one for example. You'd need a data plan for the old phone. And there are cheap wide angle and fish eye lenses you could put on the camera iPhone.
I tried the phone as camera powered by a 20,000 mah backup battery as a test at my house, but it only stayed charged a few days. The camera and app worked well however.
My Basecamp will have 160 watt solar, but I'm not sure about running the camera on the trailer battery for extended times. I'm new to the travel trailer world and assume it's not great to leave the battery connected when not in use.
For your backup camera the iPhone plan could work on its own battery. You could use a suction mount to hold it. Of course that's assuming you have an old iPhone (or Android) laying in a drawer gathering dust.
Fyi here's the Nubo...