While the thought of winter camping (I am in Wisconsin) with the current forecast makes me shiver to even think about it. Maybe I can find a 25 ft. Prka for the old camel.
I added a second battery to mine last year. I also have a furnace that uses about a third of the poer of the first one. Over all I like having the two battery set up. Mine are just group 24. A couple of 27's and you would have a 100+ amps available without falling under the 50% line. I only have 170 total (about 85 available==50% rule) and I have not come close to hitting the low level, even on a couple of nights with the furnace running consistantly a 36 led reading lamp and a radio going--maybe another light on and off here and there.
I suggest doing an amperage budget first. Look up the amp/hr of all of you appliances and you will have an idea if you need a second or not. The big 27's are expensive. My 85 amp Delco AGM batteries were priced right---free!
Guess what powers air raid sirens....only one battery is certified----guess what they do with them after a couple of years. Solve that and you are on your way to a potential source of excellent batteries (after all of the electricians have one for the trolling motor).
Back to my original point: something like one of those 12v
christmass tree type bulbs use about 1.5-1.6/hr. Your furnace fan should be on the spec sheet. Would you have a vent going etc etc etc. Do a list and think cold camping, warm camping and hot camping-all in a dry camping format.
Try to keep the battery over 50% charged, so you have 50% amps. If you have a group 27 battery you should have about 50 available amps from a full charge. A group 24 (about the size of a typical car battery--but much heavier) you will have about 40-45 amps available. It is sort of a fun project. Watch out for AC stuff on an inverter, those can really suck power. The inverter is costing you a percent or two just being on line. Do the power conversion on those appliances if you anticipate using something AC on an inverter. There are a lot of good threads on doing that and some reference to electrical budgeting charts. It is sort of fun and give you a lot of respect for power budgeting and a good generator.