I'm a sailor, not an RVer, but our needs are much the same. LiFePO4 batteries should not be charged below 32 F. However, you can discharge the batteries down to -4 F (some manufacture's have slightly different specs.) You will get 40 to 60% of the rated capacity at such low temps. What I've done is install a low temperature cut-off relay that opens at 35 F, but with the contacts bypassed with a 200 amp schottky diode to allow discharge current to bypass the contacts. Once the cabin is warmed up, the relay contacts close and charge current can flow. I am running the 100 AH lithium battery in parallel with a 100 AH AGM, so the charging circuit is not running open-load when the relay is open. The lithium battery does nearly all the work, the AGM is just there to take up the slack when the lithium is off line. My low voltage disconnect is set at 12.8 volts, and charge cutoff is set at 14.0 volts, with a restore-to-load voltage of 13.1 volts measured at the AGM. This is a very gentle cycle-range for the lithium.
There is a lot to learn when considering lithium batteries. The most important points are: 1) Getting maximum storage life requires the opposite storage-charge treatment. Lithiums last longest when stored nearly empty - while lead-acids need to be stored fully charged to prevent sulfation. 2) Because of item 1 above, trickle charging lithium batteries while in storage is a very bad idea. 3) BMS systems integrated into lithium drop-in batteries are set up to use 90% of the storage capacity. If you use external controls, like I've described, that are more conservative, you will get much more life out of the lithium battery while not sacrificing much capacity. I use 80% of capacity, and expect to double the cycle-life of the battery. 4) If conditions become extreme, your lithium battery will drop off line, potentially leaving you in the dark, or worse, causing transients from your suddenly-unloaded alternator. I suggest having at least a small AGM battery upstream and in parallel.