You have mentioned 'blow air', therefore you are considering some form of fan. This is likely to flatten the battery faster than the heat output, so you are screwed before you start.
Basically the low temperature is not the problem. It is anything in the van than is likely to suffer as a consequence, so removal of the majority of items, such as upholstery etc will ensure your van stays fine through out the winter. Visitng the van at least once a month will also allow you to ensure evrything is fine. In the spring re-install everything and take the van out for a weekend to restore the 'liveability' factor.