Looks like LFP owners should have a 10 amp ordinary car battery charger along as part of their kits. Often recommended anyway in case of converter failure so you can keep on camping.
Procedure for recharging LFPs that are below freezing:
1. heat them up to above freezing
2. charge at 10 amps till they are above 41F (charging will continue to heat them up) Leave heater on if gen can do both.
3. swap over to your high amp charger once they are above 41F
Beware of your solar amps, might have to switch off solar.
If the LFPs are outside on the tongue and never get above 41F, get more gas for your generator to keep charging at 10 amps for the month of Sundays that will take.
Have a way to measure your LFP battery cell temperature (as mentioned by itinerant1)