I would pass on the auxiliary heater for the needs you state. It is just another piece of gear to haul, will need some type of propane plumbing, and some of them emit bad stuff into the cabin air. The Olympic heater that has an exterior vent (Platinum, IIRC) is the only one of these stand-alone heaters that is externally vented. They do make wall-mounted propane furnaces for the mobile home industry but I don't think there's any savings there and they require 120V AC.
An additional 12V 125 AH battery shouldn't cost $250, more in the neighborhood of $150. Having two of these would ensure you can operate all your 12V devices through the night and would ease your generator use. If it would mean an every-other-day generator use rather than every day, an additional battery would pay for itself very quick.
At some point, you may tire from hauling generators and gas cans, listening to the noise, and being a slave to your batteries. That point is called "looking at solar". Many of us have installed solar and really like the seamless way it handles battery charging. It can also offer the opportunity to run 120V AC devices using an inverter. I have a small residential fridge and it is powered by a single solar module.
