Is it possible to combine multiple 5V USB batteries to power a Raspberry Pi for a long period of time (months)?
A Raspberry Pi model B draws about 1.21W with nothing plugged into the USB ports. 

So (say) 3 months is about 2200 hours, so you're talking about 2.6kWh if you want to power it continuously. 

A full-size laptop Li-ion battery pack might hold 48 to 80Wh, so you'd need at least 33 to 54 of them. 

A 76 lb deep cycle battery might hold 1.2 or 1.3kWh so you might only need a couple of them. 

A 5.2Ah USB battery pack might supply 25Wh so you'd need almost 100 of them. 

Doesn't sound very practical at all, I'm afraid. 

A properly designed low-power microcontroller device can run for several years on a small button cell, waking up briefly and doing things once in a while.