No, you cannot get by without killing your starting battery. You won't kill it right away, but you will drastically reduce the life of the starting battery. The starting battery is designed for one thing, to provide large amounts of current all at once, and not for low amp draws over long periods of time. If something happens to your aux battery you can connect the Espar to the starting battery as a temporary solution, but only for a few weeks at most.
Think about how a battery works. When it's cold outside, like in the winter, the starting battery already has fewer amps to begin with for starting the engine. If you run an Espar off the starting battery while it's cold outside, you're drawing even more amps from a battery that is already thin on amps, reducing the number of available cranking amps even further.
You don't need some big fancy schmancy high dollar deep cycle aux battery. A simple "marine" deep cycle hybrid battery will do, since those batteries are designed for low amp draws (running lights and trolling motors) of exactly the same type of low amp draws of the Espar.