If you are not using the AC or microwave (or other heat producing appliance), then do you really need more power at all? Maybe just a bigger capacity house battery will work fine.
Before putting money out on back up power, I would do a test run in the driveway or a local campsite with electrical. Just don't hook up the electrical and see how long you can go over a three day weekend with just the house battery. THEN you can decide if spending money on more power is needed or not. And how much more power you need.
For me, my RV fridges don't use any electricity when running on propane, so my only electrical use is the pump, lights and charging electronic devices. I can go 16 days without running low on battery. I run out of fresh water and gray/black tank space long before my electricity.