The characteristics of deep cycle batteries are entirely different to starting batteries. Deep cycle batteries are designed to be heavily and repeatedly discharged whereas starting batteries are designed to provide a very high current for a very short period ( seconds ) and only discharge by maybe 5%. Unless you make VERY light use of your auxillary items ( lights, fridge etc. ) and never forget to turn anything off, a starter battery will be likely to have a very short life.
Nowadays companies like Optima supply deep cycle batteries that can also function as starter batteries at a push ( always nice to know you have a backup ). Personally I bought an Elecsol battery ( Elecsol Batteries ) which have great functionality but don't break the budget quite as much as Optima.