The 30 amp fuse will only restrict the current. Try to pull more than 30 amps and it should pop. It won't limit you to 30 amp hours.
You really want to go to Deep Cycle or Marine batteries. Automobile starting batteries are made for a large rush of current (starting) then a quick recharge. They will die quickly when repeatedly cycled in a RE system. Deep cycle batteries are designed to supply a smaller current but with many discharge/recharge cycles. Note, even with Deep Cycle batteries, you do not want to completly discharge them. If you use no more than 30% of their capacity you should get many years of life out of one. For more capacity wire 2 or more in parallel (plus to plus to inverter, minus to minus to inverter)
Solar panels are fun, but your batteries are the true heart of your system. I suggest running a search of this board on "batteries" and spending an hour reading the results. There is a lot more out there than my brief overview. Good luck,
-Henry
