Good information, options, and recommendations can be found on the Mobile Internet Resource Center (MIRC) site.
https://www.rvmobileinternet.com/gui..._RVs_and_Boats
The above notwithstanding, considering your the requirements (affordability, zero monthly data cost when not traveling, gear that can pull in cell signal in boondocking locations) involves a number of dependencies enumerated by MIRC:
- Location of the cellular tower relative to your location
- Weather
- Tower Use Demand using the tower
- Local terrain - mountains, valleys and even trees
- Nearby buildings
- The structure of your RV
- The antennas and modem built into your iPhone cellular device
- Terms of your data plan - particularly domestic roaming limitations
My own experience over the past 24 months reflects pretty good coverage by our cellular service provider T-Mo. I had augmented this with the use of Visible (Verizon LTE) and a portable router for those occasions where a T-Mo tower was just not available and Verizon LTE tower was reachable. This happened exactly once over more than 30 consecutive days of traveling in the Northeast (MD, PA, NY, VT, ME, and NH). Somewhat more often, we were required to use the free/limited data roaming function (peer agreements between cellular providers) when no T-Mo or Verizon LTE tower/service was reachable. Once that 200 MB of domestic roaming data had been reached, no further access was possible until we reached a T-Mo tower (or the next billing cycle started). It wasn't a question of weak signal, but no signal. Clearly, in the former case, if you have signal, you can amplify with with a booster or larger external antenna. MIRC has recommendations on these. To the extent possible, using a coverage map, or perhaps even better, one of the coverage apps recommended by MIRC, you may be able to better, more specifically, match the challenges of a particular boondocking area with the signal and data plan best suited.
Apologies for my long-winded response, not sure if I've been the least bit helpful; but best of luck in your search for a satisfactory solution.