Furnace Type and Battery Set up
I have an Atwood 34000 Ducted in a 22' (27' with the V Continental Cargo). The furnace that Darko uses in his 16' is also ducted. The furnace you are looking at is a direct discharge. I prefer being able to duct as it is nice to direct the heat down and run ducting to the back of the trailer too. They are only +/- $100 more. 20-30,000 BTU will be lots for your trailer. I would look at Atwood Excalibur 8500IV 25000 BTU or the Suburban SF-25.
For battery power most of use us 2-6 Volts tied together, as they last 3 times longer than a 12V. Hook the positive post of one 6-Volt to the negative of the other 6V, and it becomes a 12 V. Then we use a normal battery charger when we are able to plug in, but with this battery set up the furnace should run at least 3 days without charging.