Go to a mobile home supply or buy a used mobile home furance, one of those stand up kerosene ones and put a tank on the outside of the garage and just leave the furnace set for 55 all winter and you should be good to go.

 

I put a hotel heating and air unit in a condo garage a few years back. It worked great and no gas line to deal with.

 

I use a furnace from a mobile home it works great.