120v DC to 5v DC conversion
Someone suggested using a normal USB wall charger directly. Would this be a possibility?


It is a possibility but a little unlikely. 



Figure 1. SMPS diagram.

Most SMPS use the configuration shown in Figure 1. At first glance it would appear that since the PSU has a rectifier anyway that you should be able to connect DC either before or after the rectifier. The problem is the DC voltage: even a "universal" 100 - 240 V AC PSU running at lowest input voltage of 100 V AC will have a peak of 100√2 = 140 V on the DC bus. Your 120 V battery might be just too low to work.

If you can find one that will work down to 85 V AC (-> 120 V peak) you might get by. Some laptop chargers will do this and you could consider this as a first stage voltage reduction into, say, a 19 V to 5 V converter of your own design.

As suggested in the comments, you could design your own. 120 V DC is in the dangerous category, however, so a pre-fabbed solution would probably be better.