What were the camels' humps good for back in the polar areas?
In my opinion there might be two reasons why the camel hump (rather than bump) might be one of the adequate adaptations of camels to living in the cold (additional to their flat feet giving hold on both snow and sand and tooth structure, Rybczynski et al., 2012). Both match the humps being fat storages in modern camels. 

The first is also provided by Rybczynski et al. (2012):


  Their iconic hump(s), containing fat, also may have been adaptive. As seen in high-latitude ungulates today (Parker et al., 2009), fat deposits could have been 
  critically important for allowing populations to survive and reproduce in harsh climates characterized by 6-month long, cold, winters.


Note that this refers to the nutritional value of the fat deposits and does not necessarily relate to maintaining body temperature.

The difference to modern camels would be that the fat storage in modern-day camels serves indirect water deposition (water can be easily gained from the fat stored in the humps and storing the water condensed in fat molecules is way more efficient than storing water directly). This relates to the second reason which, however, is more speculative from my part.

Cold - especially in plants - also implies drought as frozen water is not available. This does not need to apply to mammals as they can thaw ice, e.g. by licking. However, this might be critical as the body might lose some temperature during that process. Accordingly, fat storage might already then have been a means to storing water while keeping body temperature. 

Alternatively, and maybe even more likely, the camel hump might have undergone a switch in function from storing a food source in archaic camels to storing water in modern camels as both can be achieved with fat deposits due to the chemical properties of fat.