I charge the batteries with a 400 watt solar system and keep warm with a Olympic Catalytic Safety heater.
SunElec.comCall and ask about 12 volt panels, and try to get some in the 100 - 150 watt range.
Home Depot sells 2" angle aluminum, and cut into 6" lengths, and drill 3 each 3/16" holes for #10 screws into the roof, and 5/16" hole for a 1/4-20 bolt to the solar panel frame.
Your trailer will use 35 amp hours per day to run the CO meter, propane detector, and refrigerator. That is about what one of my 120 watt panels will put out in 24 hours.
Usually if you are running a generator, then plug in the RV and it will charge at about 45 amps for a few minutes. IT will slowly go down in amperage as time goes by. It would take several hours to fully charge a battery, so that is why solar makes so much more sense. You can use the generator to get the battery from 50% full to 75% full, then use the solar to top it off.
Fred.