In reply to ben b:
If budget is the main factor and you have transport then you could look second hand, they go cheaply on e-bay because they are heavy and hard to move. Obviously you will have to make sure they are up to the job and may have to fix a few fire bricks and replace door seals. Otherwise, Machine Mart always have cheap wood stoves for sale. Probably not the highest quality but they'd come with a warranty and would probably do the job. One issue I could see is that the smallest general stoves are usually +4 kW, that's a very warm caravan.
Alternatively, you could consider a cheap Chinese Diesel heater and run it on heating oil (essentially Diesel) or red Diesel if you can get hold of it. It doesn't have the idyllic cosy feel to it but it has to be cheaper than burning gas and much easier and cheaper than the initial install of a wood stove. Running costs obviously depend on whether she has a free supply of seasoned wood to burn.