How to ferment dosa batter?
I found some pointers here: 
http://www.indiacurry.com/south/batterexplained.htm

(The following is just taken from the information in that link, I do not actually know anything about dosa)

It seems that the lack of fermentation could be due to a number of things:  


Overwashing the ingredients (removes the wild yeast).
Using chlorinated water (kills the wild yeast). Use bottled water instead of tap water.
Iodine in the salt can also kill the wild yeast. Use kosher salt instead. 
Temperature. As you have already noted in your question, normal room temperature in the US may be too low. You could try putting it in the oven with just the light on, no heat. Or if you have a gas oven, with just the pilot light on.