Powering a sensor with a battery
The way to approach this is to figure out how much energy you need, then see if there's a battery big enough to support it.

You need 60mA for 720 hours (24hours*30 days.)  

That is 720hours*0.06 Amperes= 43.2 Ampere Hours.

Since your sensor can operate on 12Volts, you need a 12Volt battery rated for 43 AmpereHours (Ah).

A quick look around google says that batteries of that size are used in cars and trucks.

A battery pack like you would use to recharge a phone via USB provides 5Volts, so it won't work without extra parts (a boost regulator to transform 5V to 12V.)  It also won't have the needed capacity.  

Powerbanks are rated in milliAmpereHours.  To compare to your needed capacity, you have to multiply by 1000.  So, you need a powerbank rated for 43000mAh.  

Well, no.  You need more than that because you have to trade current for voltage to get the 5Volts from the powerbank up to the 12V needed for your sensor.  So, at least (12/5)*43000=103200mAh.  You won't find one that size for a price you will like, and maybe not at all as a separate unit.

So, your best bet is to use a fully charged 12V auto battery of at least 43Ah capacity.



In response to the comment from @Ecnerwal:

A battery made for deep discharge (such as a trolling motor battery) would work better.  Trolling motor batteries are available at 12V.  The difference is that trolling motor batteries are designed to be discharged to the limit without being damaged.  A regular car battery used this way might not be able to hold a charge properly after being run down - you would have to replace it.  If this is a one off operation and you can get a car battery cheap it might not matter.  If you will need to use the battery again you might want to look into getting a battery rated for deep discharge.



You might get away with less if the sensor shuts down most of the time and only draws 60mA when transmitting.  You didn't say, and maybe you don't know.  The data sheet for the sensor would probably say, so it would be nice of you to post a link to the datasheet for the sensor.