I just read this thread, and did some searching. I found this device.
HomeSitter Temperature, Water, and Power Alarm (HS-700)
A light went on!!!
Doesn't happen often at my age.
Somewhere around here, I have a device that makes devices like this think that a cell phone is a home phone. If I can find it, I am thinking that we could leave one cell phone(we always have 2) in the RV and connect this monitor to it. We could configure the monitor to call our other cellphone. To clean it up we could forward incoming calls directed to the phone left in the RV to the phone we take.
This would eliminate the need to pay an addition cell line charge.
If I get anywhere with it, I will post the info here. If anyone else tries this or something similar let us know.
The device I have is this or something similar.
Her's another link if you want to research some other products.
My conclusion after reading the above chart is that there are many ways to accomplish this including landline, cell and wireless. One could also leave a cell phone in the trailer configured as a wifi hotspot if one buys a monitor that interfaces via wifi.
If we got one that interfaces via wifi, we would not have to leave a cell phone, because we also have a Verizon tablet with a WIFI hotspot.