Your post looks sloppy and lazy, making it difficult for anyone to help you or to want to help you. For instance, I suspect you are not actually trying to run a 12 V cell-phone charger off of your phone battery. But even if I assume you want to make a portable power pack into which you can plug in your phone charger, I still cannot really understand your question. What are you trying to achieve? Why do you care about efficiency? Typically, in an energy conversation, the relevant efficiency is how much energy you spend charging your power pack vs. what you get back out of it, but it seems like you might care more about being cost efficient or size efficient.
It is not appropriate to think about an "is it okay" question based on the specs you have given. You can use the current and voltage specifications of the chargers to get a rough best-case estimate of how long it will take to charge the batteries, but whether it will work at all or be safe is a completely separate issue. You should only use chargers that are specifically made for your kind of battery pack (and you should make sure you know how to configure them appropriately).
Sorry, I am not familiar with those Kyosho 4-wire servos. By the way, once we are dealing with a proprietary system like this, we don't know what the first 3 wires are, either (though power and ground are good guesses for two of them). If you (or anyone else reading this) have a broken one that you would be willing to send in, I would be happy to take a look at it.
More Ah will just last longer, and since you don't need much, it seems like the primary consideration should be size or weight. Separately, though, I think this 12V battery->inverter->AC adapter route is quite awkward. You can probably get by with just using AA batteries. You could either set up your own battery holder or get some AA to C battery adapters, which are just sleeves or shells that fit around AA batteries to make the final diameter match that of C batteries.
You start out ok, up through where you got the 20 mA, but then you got sloppy with the units in exactly the way this blog post was supposed to help people avoid. 20 mA is already a rate, and as long as that fan is running, the current is 20 mA, and it is wrong to say 20 mA per hour or 10 mA for 30 minutes. It would be correct to say the battery will deliver 10 mAh in 30 minutes.
I've crossed out the incorrect parts in your comment in the hopes that it will help others avoid this kind of mistake. The 65 hour result based on the starting assumptions is right, though.
As a rough approximation (within a factor of two), I would expect similar battery life. The main thing I'm skeptical about is that 5V is not a natural voltage for a lithium ion battery. If there's actually a 3.7 V, 2600 mAh battery inside and the converter to 5V is 90% efficient, you'd be getting the equivalent of about 5V, 1750mAh.
I don't know enough about your system to know for sure. There's got to be some limit on your alternator and whatever battery charging circuit is in your car, and while my impression is that lead acid batteries are among the more forgiving regarding charging, you still might limit their life or otherwise damage them if you just do what you are proposing.
I am concerned that with the level of electronics understanding you have presented, you might be risking damaging or destroying some fairly expensive equipment. It seems like you might be better off looking for some pre-made solution, like a general-purpose external/backup battery for phones or other electronics.
On to your specific case. First off, you should realize that with four batteries instead of the built-in two, you will get at most 2x the battery life. You should not put your batteries in parallel, and if you just put all your batteries in parallel, you would only have the 1.2V individual battery voltage anyway. You should also go through a regulator, too, to make sure you actually give your device the 5V it is expecting since the fully charged batteries might be 5.5V or more and damage the device, and they might get too low as they discharge. Something like this step-up/step-down regulator could work:
With that regulator, it would be better to go with 6 AA batteries if you have the room for it. But, if you go down this path, make sure you get a better understanding of what you are doing and that you are ready for the consequences if you make a mistake.