Well, if you are certain you wish to use an SLA battery, then a 'buck converter' is what you require. This steps down a voltage quite efficiently (80-90%).
As for preventing the system draining power form the battery when the camera is off (ie: its not drawing power) then a simple switch between the battery and your regulator circuit??
Battery life of your current AA's (alkaline) depends on not only the current draw, but also the minimum voltage the camera can accept. Alkalines are generally considered 'dead' when they get to 1V...and so their mAH ratings is until they get to 1V...I doubt your camera can accept that and so would probably consider something like '1.3V/cell' as 'dead'. Also battery capacity depends on current drain...alkalines have lower capacity for higher current drains.
Not trying to confuse you, just saying...its a bit 'ish' to judge battery life of alternate batteries.
My suggestions for an external battery pack:
Battery type: NiMH tend to have a better 'capacity/weight' ratio than SLA's, Perhaps 4-6 D size cells would be ok? But Of course, cost is a consideration. You will have to decide on weight, size, capacity.
Regulator: in order to get the voltage your camera wants (2.9-3v?) From another voltage source - usually higher voltage, but using a lower voltage is also possibile - I can see two options.
Linear regulator - will waste power when on and when the camera is off. Unless the battery you choose will hold just over 3v over its discharge..then even an LDO regulator would be quite inefficient.
Buck regulator (switching regulator) Quite efficient, and usually has a 'power down' mode that will draw only uA from the battery when not in use. Check ebay for MC34063 modules - premade.
Well I hope thats food for thought at least.
Blueteeth