This is a good plan but the only downside is that you will need some mechanism to hold back the charging until the batteries get above 35F (or whatever you decide the safe to charge temp is).
That’s the job of the BMS. I am using the Overkill 8 Cell model, and you can set both temp for charging cutoff and for resumption. So, right now, the low temp disconnect is set to 32°F and the reconnect is 37°F. The heaters will come on at 35° as long as there is amperage from solar.
There will likely be some fine tuning, but that’s what I’m starting with.
FYI - for the last couple weeks the Solar Charge Controller (Victron SmartSolar) has been trying to send charge through the BMS which is below cutoff temp, and nothing seems to have suffered any damage.
I plan to set the "discharge not allowed" signal once under the safe discharge temp to disable all loads as I presume any humans or pets in the RV are now frozen and do not need power.