I have not read every post in this thread, but I do believe I understand what you want. I also share your desire to some degree because while I am not a Full-time RV’er, I do own one & spend approximately 2 months a year in it. I am going to list my assumptions of your situation & possible solutions for you. Please correct anything that I get wrong. And anyone else please correct me if any of my solutions are not valid.
First, you have stated you pay a lot of money for data. I am going to assume that this is an unlimited plan with a cell phone company (hopefully it is Verizon or AT&T for they have the best networks) and you own a smartphone.
Next, you have stated you are a Full-time RV’er. So I am assuming your home base is where ever you have parked your RV. And that you seldom have internet access & when you do it is not meant for heavy use as in streaming video.
Lastly, I then conclude that if you have or plan to purchase a Tablo that it will always be with you & connected to your RV antenna.
If all those assumptions are correct then whenever you are in a location that has both OTA signals & cell service you will be able to setup a hotspot on your phone for internet access & watch & record OTA programming using very little data. The data usage will be small because you won’t be using remote access but rather it will the same as for anyone using a Tablo at home. Furthermore, anytime you are at a location that has cell service you will be able to watch any recordings you may have even when there is no OTA signals available.
I apologize for the length of this post & hope that maybe I gave you some info you can use.