A few things about adding batteries.
When adding a matching battery, you don't want the older battery to be very old with diminished capacity. It can drag down the new good battery. They should at least be of the same size and amp hours.
When adding extra battery somewhere else in the rig, you need to keep the runs of wire close to the same for both banks so that they are balanced. Make sure you use a heavy enough wire for the distance.
Sealed batteries are fine inside the rig but flooded batteries have to have ventilation.
When dry camping, conservation is as important as adding extra capacity. We currently have a single grp 27 battery with 95 AH and even though we run the furnace nightly in temps as low as the 20s, we have never dropped down to 50% of capacity, much less drained the battery over night. We do this by using as little power as we can without feeling like we are still tenting. Things like...
We replaced all of the lighting with inexpensive ebay led lights for $50. We now use the lights at will and never worry as they use a 5th of the power the regular bulbs do. We use to not use the house lights at all, instead used HF puck lights at night.
We turn off the hot water heater and pump when not in use. We keep a water jug in the trailer for small uses like coffee so we don't need the pump and it doesn't wake honey up at 5 am. On really cold nights, we even shut down the fridge.
Anything that can be charged by 12v gets charged in the truck over night. That includes cell phones, computers, mp3 players, portable DVD players and small TVs. I have never drained our trucks battery using it for charging.
At night we seal the windows with Glad press and seal and then drape heavy bath towels over them. This cuts down on heat loss and the furnace runs much less. We set the furnace at 65 at night.
Some people can use portable propane heaters such as the Mr Buddy line or Olympic wave because they don't use the battery. I can't because of the altitude we camp at but it is a option if it works for you.
The last thing is to have a way to keep the battery charged. We currently have 3 ways to do this. Solar, a Generator and as a last resort, jumper cables to the truck idling for a hour each day. Batteries do best when not left discharged for long periods of time.
