Washing out used milk jugs and filling with water can soak up the heat during the day or any warm time and then they will slowly release the heat back into the room over time thus helping to stave off cold returns to the RV and prolonging the heat overnight when the fire goes out in the woodstove. Since you are not using your RV shower, this would be a good place to store many milk jugs stacked on shelving made from pallets as well as the area opposite of your bed. This would also give you a large amount of water storage since having to drain your outdoor tanks. You can place them throughout your RV(half gallon jugs can be more manageable in smaller places like under countertops). Placing them in the front windshield of the RV so they can soak up the sun could be a good place(painting them black probably wouldn't hurt either).
So many people drink milk these days it shouldn't be to hard to find plenty of them by asking around(especially people with small children as they drink alot of milk). Using water as a heat storage works good but remember, the larger the tank the longer it will take to loose all of its heat. i.e. a 20 gallon tank will loose all of it's heat slower than 20 1 gallon jugs. But the more water you can store inside the RV the better your RV will maintain a warmer temperature.
Used milk jugs=warmth+water storage
Re: Used milk jugs=warmth+water storage
Yes water can retain a lot of heat. Next year when/if I build a cottage I am thinking about bringing my rain water tank inside the cottage and putting it next to the wood stove. I can maybe even use a sort of heat exchanger to keep it warm. Just a bit above room temperature would be good enough to help maintain indoor heat during long periods when the stove is not burning