One older battery in a set will drag the others down. Spend the extra bucks and replace all four. In a general sense you can mix old and new IF you use same brand, same aH and the older batteries are in good condition.
Put a battery watering system on them while you got them out. Flow-Rite is one, at Amazon or auto parts store. Simple to install and use. I water mine once per month.
Not knowing how much dry camping you do go with the GC-2 from Costco/Interstate with 200 aH each, about $100 each. Lots of dry camping and the deeper pockets you might want to spring for the Trojan T-105's at 225 aH. Usually turn on generator in the AM for coffee and the microwave and leave it on for 2-3 hours.
We do about 2-3 days at a time, residential fridge, temps down to mid to upper 30's and the batteries last overnight with furnace use at 60°, usual TV and LED lights.