batteriesremote control
Pretty much all BECs (Battery Eliminator Circuit) are rated to work fine with 12v input but I haven't seen any that mentions lead acid batteries. Almost all datasheets for BEC regulators have write ups such as:
Input voltage: 6V to 33.6v (2s-8s lithium, up to 21 NiMHs)
Is there any concerns using these devices together with a large 100Ah Sealed lead acid (marine) battery?
There are many power supplies available for this purpose. As 0x6d64 says, these have a constant voltage output of around 2.3V per cell, ie 13.6V or 13.8V for a 12V battery. The power supply will need a current limit so that the battery will not be damaged if it is charged when flat. This would typically be about 0.1 of the capacity - so no more than 10A for a 100AH battery - probably 5A to be safe.
You can safely leave everything connected 24/7 with this arrangement and switch the mains on and off as necessary but some deep-discharge protection is advisable. This would cut the power output if the battery voltage drops below about 10V to prevent battery damage. Of course your connected equipment needs to be capable of operating at 13.8V for this to work. If 14V is the absolute maximum then this solution is borderline.
Firstly, the feed to the cameras: I think, to be safe you should use a low drop-out regulator to feed your cameras - this takes care of slight overvoltages. The KA278R12C is a linear voltage regulator with very low drop out: -
Note that even when the input voltage is at 10V, the device is still able to ostensibly produce 10V at its ouput when delivering over an amp (6 ohm load). I suspect this device will be good enough to feed your camera system but I can't absolutely say because you haven't specified current. There are other higher power devices that would fit the bill.
Can I wire a load to my battery if it is connected in parallel with the charger?
If the battery is lead/acid and the charging current is significantly more than what the camera load takes when attached to the above regulator then yes you can. If the battery isn't lead-acid then we need to know which technology it is.
How can I add a solar panel + controller to the previous circuit?
Playing safe, you can use a relay circuit that activates the relay when the AC power is applied to the charger - the relay contact can switch the battery from solar charger to AC charger in a few milli seconds. Playing a little bit unsafe, it's likely that your solar charger will have a diode in its output that protects the battery from discharge when the sun doesn't shine.
This very same component probably can mean that you can connect the AC charger permanently to the battery (and solar charger) BUT, you may need to add a series diode\$^1\$ in the AC charger's output when AC is off and the solar charger is feeding juice to the battery; the AC charger's output circuits may be activated by the solar charger and it's difficult to say what will happen - worst case it might pop the output transistor in the AC charger - best case no problem.
However, the chances are likely that your AC charger (just like your solar charger) will be protected from reverse voltages when power is down (or sun is not shining). You need to check this.
\$^1\$The diode needs to be a low volt drop schottky type capable of taking the charge current (again, you haven't specified max charging current so it's impossible to say but there are plenty rated for 10A and 20A continuous usage).
Best Answer
As long as the input voltage is in the specified range, the BEC does not know or care what kind of battery is connected to it. A BEC should work fine with a marine lead-acid battery.