You seem set on a different heater so I'm probably wasting my time but I will reiterate what's already been said. Get another 12 volt battery and you will be fine. The heater that comes in most RV's are not that bad considering you are heating a big box with very little insulation and big square holes in the roof covered with thin plastic lids. If you plan on doing more dry camping your money would be better spent on a battery and some solar to keep the batteries up.
This time of year your heater will not run very much at night even if the temps do get down in the 30's as the trailer will probably get heated up during the day and take a while to cool down at night. Secret here is to manage air flow. When the outside temp warms up open up the trailer to let the warm air in so the trailer warms, as it starts to cool in the evening close the trailer up to hold the warm air. If needed before bed run the heater at 70 for a couple cycle to warm everything up then set the thermostat at 55 or 60 for the night.
2 12 volt batteries, 200 watts of solar in my 30 foot toy hauler keeps my batteries alive for 2 weeks of boondocking in temps ranging from 40 during the day to below zero at night.
