Yes, my cell phone has "hotspot and tethering" but it lies! It actually has Bluetooth or USB tethering. No hotspot relay as such.
So the USB part works great to my laptop, EG. I can get free hotspot wifi on the cell (or my camera signal), and leave that alone and run the laptop as though it had that hotspot connection.
So you could do that if you had a USB cable that has the plug in for the phone on one end and the different Apple one on the other end or an adapter for that, and long cable run (end to end sets) from the cell in the front of the trailer to the truck's mirror. (Might as well buy a wired camera?)
I have looked at wifi extenders (lots of those) but can't make out how they can pick the right wifi signal to extend by wireless and then relay that to the farther along device. Some(all?) have LAN from the extender back to the first wifi source box and then they wifi from there, (I think)
I am waiting to hear from Hopkins how you can do it. I suppose any 13 year old knows, but I don't! :) I am just curious anyway, I don't need that in my set-ups.